Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Tree mortality and regeneration of Euphrates poplar riparian forests along the Tarim River, Northwest China

Tree mortality and regeneration of Euphrates poplar riparian forests along the Tarim River,... Background: Tree mortality and regeneration (seedling and sapling recruitment) are essential components of forest dynamics in arid regions, especially where subjected to serious eco-hydrological problems. In recent decades, the mortality of the Euphrates poplar (Populus euphratica) along the Tarim River in Northwest China has increased. However, few studies have quantified the causes of mortality and regeneration in this azonal riparian forest type. Methods: The present study describes the annual hydrological response of tree mortality and regeneration in forest gaps. A total of 60 canopy gaps were investigated in six replicate grid plots (50 m × 50 m) and the annual runoff and water consumption data during the period of 1955–2016 were collected from hydrological stations in the middle reaches of the Tarim River. We compared the regeneration density of seedlings and saplings within the canopy gap areas (CGAs), undercanopy areas (UCAs), and uncovered riverbank areas (RBAs) through detailed field investigation. Results: Our study found that the mortality of young and middle-aged gap makers has increased remarkably over recent decades, particularly since the year 1996. The main results indicated that regional water scarcity was the primary limiting factor for long-term changes in tree mortality, as shown by a significant correlation between the diameter at breast height (DBH) of dead trees and the annual surface water. The average density (or regeneration rate) of seedlings and saplings was highest in the RBAs, intermediate in the CGAs, and lowest in the UCAs. Compared with the UCAs, the CGAs promote tree regeneration to some extent by providing favorable conditions for the survival and growth of seedlings and saplings, which would otherwise be suppressed in the understory. Furthermore, although the density of seedlings and saplings in theCGAswas notashighasinthe RBAs, thesurvivalratewas higher in the CGAsthaninthe RBAs. Conclusion: Forest canopy gaps in floodplain areas can play a decisive role in the long-term germination and regeneration of plant species. However, as a typical phreatophyte in this hyper-arid region, the ecosystem structure, functions and services of this fragile P. euphratica floodplain forests are threatened by a continuous decrease of water resources, due to excessive water use for agricultural irrigation, which has resulted in a severe reduction of intact poplar forests. Furthermore, the survival of seedlings and saplings is influenced by light availability and soil water at the regional scale. Our findings suggest that policymakers may need to reconsider the restoration and regeneration measures implemented in riparian P. euphratica forests to improve flood water efficiency and create canopy gaps. Our results provide with valuable reference information for the conservation and sustainable development of floodplain forest ecosystems. Keywords: Tree mortality, Regeneration strategy, Seedling and sapling recruitment, Gap makers, Riparian forest, Tarim River * Correspondence: halik@xju.edu.cn College of Resources and Environmental Science, Key Laboratory for Oasis Ecology of Ministry of Education, Xinjiang University, Ürümqi 830046, China Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Keram et al. Forest Ecosystems (2021) 8:49 Page 2 of 13 Highlights ecosystem balance (Wang et al. 1995; Huang 2002; Huang and Pang 2011; Mamat et al. 2019). Nevertheless, Effect of hydrological alterations on tree mortality of the Tarim floodplain ecosystem, especially the riparian Populus euphratica riparian forest was quantified. vegetation, has been seriously degraded, as the water The contribution of different habitats (canopy gap conveyance in the main channel has rapidly decreased area, undercanopy area and riverbank area) on plant and many tributaries of the watershed have been discon- regenerations was evaluated. nected from the main river (Zhao et al. 2011; Chen et al. Recommendations for regeneration and succession 2013). of riparian forests at the Tarim River were put Many studies have demonstrated that water shortages forward. in the Tarim are mainly caused by climate change coupled with intensive anthropogenic activities, i.e., tree Introduction felling, overgrazing, excessive land reclamation and Riparian forests provide fundamental ecosystem services disorderly expansion of cotton monoculture. Therefore, in arid regions, including maintenance of ecological sta- restoring degraded floodplain ecosystems has become a bility and prevention of natural disasters such as sand- major focus of applied forest research (Deng 2009;Rum- storms, heatwaves, and desertification (Song et al. 2000; baur et al. 2015; Thomas et al. 2017; Thomas and Lang Chen et al. 2013; Betz et al. 2015; Mamat et al. 2018; 2020). Among all processes of natural forest dynamics, Halik et al. 2019). The existence of trees, bushes, and the formation of forest canopy gaps was considered a grass vegetation on the banks of the Tarim River, which vital regeneration strategy for maintaining floodplain constitute the natural barriers of river oases, is abso- forest structures (Han et al. 2011; Keram et al. 2019). lutely dependent on the river (Zeng et al. 2002, 2006; Gap disturbance is the main driving force of forest dy- Halik et al. 2006; Aishan et al. 2018). In recent decades, namics as it creates environmental heterogeneity (Zhu floodplain vegetation has been threatened by increased et al. 2003, 2007; Albanesi et al. 2005; Bottero et al. water scarcity, and large areas of riparian forests includ- 2011). Forest gaps thus create important habitat for the ing seedlings and saplings, have withered (Gries et al. regeneration of plant species, which may otherwise be 2003; Foetzki 2004; Aishan 2016; Zeng et al. 2020). With suppressed by the undercanopy (Han et al. 2011; Sharma China’s rapid economic development and the implemen- et al. 2018). They also play a vital role in forest regener- tation of the “Ecological Civilization” and “One Belt One ation and succession, especially in the establishment and Road” initiatives, the ecological restoration of the flood- development of plant species that differ in ecological re- plain ecosystem along the Tarim River has been desig- cruitment (Runkle 1998; Mountford 2006; Rentch et al. nated as a high priority by the Chinese government 2010; Han et al. 2013; Zhu et al. 2014; Jankovska et al. (Halik et al. 2006, 2019). Conservation of the remaining 2015). Nagel et al. (2010) reported a high presence of ad- riparian forests and recovery of the degraded parts of vanced regeneration in a gap regeneration study of a the ecosystem are crucial for further sustainable devel- (mixed) beech virgin forests. Besides, it also has been re- opment of the region (Rumbaur et al. 2015; Deng 2016; ported that canopy gaps close when the height of regen- China Green Foundation 2018), particularly, transport erations reached 20 m, consistent with the definition infrastructure, including railways, highways, and oil and used by Nagel and Svoboda (2008). Zhu et al. (2021) gas pipelines in the Tarim Basin are in need to be pro- found that with the decreased of gap size, pioneer spe- tected (Deng 2009, 2016; Aishan 2016; Halik et al. 2019). cies became the sub-canopy layer, and with the aging of Thus, it is necessary to preserve the functions and ser- gap, the light conditions changed over time, which was vices of these forests through adaptive management. conductive to the recruitment of shade-tolerant species; The riparian forest (also known as Tugai forests) at Keram et al. (2019) also revealed that hydrological con- the Tarim River constitute a natural green belt at the ditions (groundwater, runoff and water consumption) northern edge of the Taklimakan Desert. The Euphrates are the main driving force of the gap-scale disturbance poplar (Populus euphratica Oliv.) is a dominant tree spe- of desert riparian forests along the Tarim River. In flood- cies in the floodplain ecosystems. As a “Green Corridor”, plain forests, canopy gaps may not be filled with regen- P. euphratica forests have become increasingly import- erations within a short period because of high tree ant for preventing the unification of two neighboring mortality. This leads to the continual expansion of can- sandy deserts, the Taklimakan and the Kuruktagh (Xu opy gaps (Keram et al. 2019). The majority of earlier et al. 2006; Betz et al. 2015). These forests have import- studies have focused on the mortality of gap makers and ant ecological functions in addition to socio-economic the diversity of regeneration species in other various cli- and touristic value, such as protection of biodiversity, matic zones, such as tropical, subtropical, north temper- regulation of the climate and hydrologic conditions in ate, and cold temperate regions (Runkle and Yetter oases, fertilization of soils, and maintenance of regional 1987; Yamamoto 2000; Dorotä and Thomast 2008; Keram et al. Forest Ecosystems (2021) 8:49 Page 3 of 13 Petritan et al. 2013; Popa et al. 2017; Zhu et al. 2018; (H3) Kitao et al. 2018). However, systematic research on tree In floodplain forests, due to the degradation of plant mortality and plant regeneration of desert riparian for- habitat, canopy gaps may not be filled up with other re- ests under various hydrological scenarios are relatively generations in a relatively short period. By comparison lacking. Therefore, it is necessary to scientifically under- of the species diversity between gap makers and gap stand the response of P. euphratica mortality rate to fillers in canopy gaps, we hypothesized that canopy gaps hydrological dynamics at regional scales, and seedling may not be filled up with other regenerations within a and sapling regeneration in the Tarim River Basin. short time, (i) which may be due to the degraded habitat Forest canopy gaps may provide opportunities for of plant growth, (ii) prolonged and unexpected flooding species regeneration and have therefore been widely disturbance resulted in the degradation of P. euphratica exploited in forest recovery programs (Coates and Philip trees, and (iii) its mortality did not cause significant 1997; Schliemann and Bockheim 2011; Kern et al. 2013, changes in tree distribution but caused changes at tree 2017; Nagel et al. 2016; Lu et al. 2018). Thus, it is neces- level. sary to conduct studies to better understand the mortal- We intend to verification of these hypotheses by com- ity of gap makers under long-term hydrological piling data from six permanent monitoring plots located processes and seedling and sapling establishment in dif- at undisturbed natural forest sites along the middle ferent habitats, such as canopy gap areas (CGAs), under- reaches of the Tarim River. We believe that addressing canopy areas (UCAs), and riverbank areas (RBAs) of these hypotheses will contribute to improve the under- floodplain, as well as to clarify the contribution of can- standing of the current state of the Tarim riparian eco- opy gaps to plant regeneration in riparian forests. Such system, where P. euphratica is now in urgent need of information may provide insights into the efficiency of protection. Additionally, it will provide an ecological gap usage and seedling planting in forest restoration ac- framework for the design and implementation of close- tivities along the Tarim River. In the present study, we to-nature restoration techniques to extend P. euphratica used comprehensive field investigation data to describe forests. tree mortality as well as to compare the density of seed- lings and saplings between the CGAs, UCAs, and RBAs Methods to evaluate whether the CGAs are important for the re- Study site generation of P. euphratica riparian forests along the Field work was conducted in Yingbazha village (41.22°N, Tarim River. In this work we made three hypotheses: 84.31°E, ASL 1000 m) in Tarim Huyanglin Nature Reserve (THNR) along the middle reaches of the Tarim River (Fig. 1), which was established in 1984 and (H1) upgraded to a National Nature Reserve in 2006. The re- Based on the observation that canopy gap disturbances gion is an extremely arid warm temperate zone, with an have frequently emerged in the desert riparian forests annual mean precipitation of 75 mm and an annual and its composition and structure were drastically differ- mean temperature of 11.05 °C (Keyimu et al. 2018). The ent from other forest types (Han et al. 2011; Keram et al. annual potential evapotranspiration ranges from 2500 to 2019). We hypothesized that characteristics of tree mor- 3500 mm (Rouzi et al. 2018). According to the USDA tality (number, DBH) strongly responded to hydrological (United States Department of Agriculture) soil classifica- dynamics (such as stream flow) at local and regional tion system, the soil of the Tarim River is a member of scales in the Tarim River Basin, resulting in the emer- the Aridisol order, and the soil is silty loam (Hu et al. gence of special forest gap structures. 2009; Yu et al. 2009). The local groundwater system is recharged from the surface water through bank infiltration (Huang et al. 2010). The forest structure and composition (H2) in the study area is relatively simple, and the local flora is Conclusions on the size development of canopy gaps, mainly composed of P. euphratica, Tamarix ramosissima once formed they would continuously expand (Keram and Phragmites australis (Table 1). However, a few rare et al. 2019). The important process of gap dynamics, plant species are also present, including Populus pruinosa, i.e. plant regeneration, in floodplain forests has not Tamarix hispida, Tamarix leptostachys, Glycyrrhiza glabra, been quantified yet. To test the general hypothesis, Inula salsoloides, Karelinia caspica, Lycium ruthenicum, that canopy gaps promote plant regeneration in ripar- Alhagi sparsifolia, Apocynum venetum, Halimodendron ian forests, we formulated the hypothesis that canopy halodendron, Poacynum hendersonii, Cirsium segetum, gaps drive tree regeneration to some degree, due to Cynanchum auriculatum, Aeluropus pungens, Taraxacum canopy gap provides more adequate light conditions mongolicum, Salsola collina, Calamagrostis pseudophrag- than the UCAs. mites, Myricaria platyphylla, Halogeton glomeratus,and Keram et al. Forest Ecosystems (2021) 8:49 Page 4 of 13 Fig. 1 Sketch map of the study area Elaeagnus oxycarpa (Gries et al. 2003;Chenetal. 2004; were recorded for each tree. Morphological characteris- Thevs 2006;Thevs et al. 2008;Huang et al. 2010;Lang tics were used to determine the degree of tree decom- et al. 2016; Halik et al. 2019). position and the life expectancy of gap makers was estimated under the guidance of local experienced forest Field investigation workers (Liu and Hytteborn 1991; Droessler and Von A complete field survey was carried out from May 15 to 2005; Thomas and Jurij 2006; Petritan et al. 2013; Wen June 20, 2017. A total of 60 canopy gaps were investi- 2016; Yang et al. 2017; Keram et al. 2019), and the rela- gated in six replicate grid plots (50 m × 50 m) (Table 1). tive ages of the gap makers were thus determined. In Canopy gaps were created by tree mortality (i.e., death) addition, we determined whether there were significant or by > 50% loss of tree branches. The ratio of gap diam- differences in the DBH distribution among living and eter to tree height on the border of the gap (R value) dead trees. We regarded trees bordering canopy gaps as D/H ranged from 0.4 to 2.2 (Keram et al. 2019), which is con- living trees and gap makers as dead trees. To investigate sistent with measurements reported by Zhu et al. (2015). regeneration species, we selected five replicate plots in For each canopy gap, all surrounding trees and gap each habitat (CGAs, UCAs, and RBAs) and recorded the makers were identified, and the species, diameter at species and numbers of seedlings and saplings. We sim- breast height (DBH), and tree height (TH) or log length ultaneously compared the species diversity and density Table 1 Characteristics of six natural stands sampled for analysis of tree mortality Plots Area DRB GWL Lat. Long. Altitude P. euphratica T. ramosissima Gap (m ) (m) (m) (N) (°) (E) (°) (m) openness Ave. DBH (cm) Ave. TH (m) Ave. DBH (cm) Ave. TH (m) (%) 1 50 × 50 100 2.00 41.175 84.240 929 35.4 9.8 1.6 3.2 43.10 2 50 × 50 150 2.68 41.229 84.333 920 21.6 12.1 4.2 2.6 66.16 3 50 × 50 300 3.41 41.176 84.241 920 15.5 9.1 2.3 1.6 50.54 4 50 × 50 600 4.55 41.174 84.237 916 23.5 11.0 3.2 2.4 41.38 5 50 × 50 1200 5.45 41.252 84.340 915 28.4 10.5 5.4 2.2 52.41 6 50 × 50 2400 7.50 41.228 84.334 916 21.1 12.1 2.6 1.7 52.12 DRB Distance from river bank, GWL Groundwater level, DBH Diameter at the breast height, TH Tree height Keram et al. Forest Ecosystems (2021) 8:49 Page 5 of 13 in these plots. We defined a CGA as an area where the method was used to statistically analyze the change canopy gap > 10 m . For the UCA, we chose plots trend of gap makers that died in different years and to around the canopy gaps in the study area; RBA plots evaluate the distribution patterns of the DBH values of were located near the riverbank, which were covered gap makers. We used a chi-square goodness-of-fit test to with excessive water. For each habitat, 15 repeated sam- reveal the DBH distribution of dead trees versus living pling plots (10 m × 10 m) were set, giving a total of 45 trees. Further, we comparatively analyzed plant regener- sampling plots. In each plot, we counted the number of ation density between the different habitats (CGAs, species, for seedlings (≤10 cm tall) and saplings (10–100 UCAs, and RBAs) using the Margalef and Shannon- cm tall) (Walker 2000). Wiener indices. Values of P < 0.05 were considered to be statistically significant. The data were organized using Data processing Excel 2015 (Microsoft, Redmond, USA) and analyzed Classifying the DBH structure of gap makers using OriginPro 2016 (OriginLab Corp2016). Over the entire area of canopy gaps, gap makers were classified into five categories according to their DBH Results and TH (Xu et al. 2016): Class I comprised juvenile trees Tree mortality (DBH < 5 cm, TH < 4 m), Class II comprised young trees Among the 245 gap makers identified, the mean DBH (5 cm < DBH < 10 cm, TH ≥ 4 m), Class III comprised was 20.80 ± 9.03 cm (range 10.10–49.50 cm) and there young/middle-aged trees (10 cm < DBH < 15 cm), Class were significant variations in different years. The chi- IV comprised middle-aged trees (15 cm < DBH < 20 cm), square goodness-of-fit test showed that the DBH and Class V comprised old trees (DBH > 20 cm). structure of gap makers presented a common curve dis- tribution (Fig. 2). The DBH distribution of gap makers Dynamic index of gap maker structure approximated a Gaussian fit (R = 0.762, 0.749, 0.829, The dynamic structure of gap makers was quantitatively 0.896, 0.918, and 0.364, respectively; P < 0.05). However, analyzed. The dynamic index of population structure large young gap makers (DBH < 15 cm) were more com- (V ) can be represented by the change trend of gap mon in the study area, especially in the 2007–2016 Pi makers in various structures. If 0 < V ≤ 1, the quantity (Gaussian fit, R = 0.364, P < 0.05). During the investiga- Pi of gap makers increased with the V value; If − 1< V ≤ tion, we found that some young trees were injured or Pi Pi 0, the quantity of gap makers decreased with the V crushed by old trees. Pi value. The dynamic index of gap maker individuals (V ) In this study, we found that stream water was signifi- without external disturbance is described by the follow- cantly negatively correlated with tree mortality rates over ing equation: the last three decades. Based on the above data analysis, there was an ascending trend in the dynamic index of S −S n nþ1 gap maker’s population structure during the periods of V ¼  100% ð1Þ ðÞ S −S n nþ1 max 1977–1986, 1987–1996, 1997–2006, and 2007–2016, while in 1957–1966 and 1967–1976 there was a where S is the number of individuals in n DBH class, descending trend (Table 2). Furthermore, there was a S is the number of individuals in n + 1 DBH class of n +1 considerable negative interaction between the dynamic gap makers, and (…) is the maximum value in the max index value of gap maker population structure and column. Below is the formula used to calculate the dy- annual runoff; the equation of the fitted model was namic index of gap maker structure (V ): Pi y = − 0.0215x + 0.6146 (R = 0.912, P < 0.01) without ex- ternal disturbances (V ), while the equation of the 1 K−1 pi V ¼  ðÞ S  V ð2Þ P 2 pi n n K−1 n−1 fitted model was y = − 0.0005x + 0.0146 (R = 0.8405, n−1 P < 0.01) under completely random disturbances (V ′) pi where S and V are the same as in Eq. (1), K is the and y = 0.0032x +0.112 (R = 0.356, P < 0.01) under n n quantity of gap makers in different age classes, and the non-random disturbances (V ′′). V and V ′ decreased pi pi pi range of V is consistent with V . We used the formula markedly with increase of stream runoff in the study area Pi n proposed by Chen (1998) to calculate the dynamic index (Fig. 3). The V ′ value became positive and tended to pi 8 3 value under completely random (V ’) and non-random increase when the river dropped to 25.86 × 10 m . pi (V ′′) disturbances. Comparison of the DBH distribution of dead trees and pi living trees showed significant variation with a high Statistical analysis proportion of young dead trees (Fig. 4). As mentioned We have calculated species diversity according to the above, the trees surrounding CGAs and gap makers Margalef richness index and Shannon Wiener index (Ma represented living trees and dead trees, respectively. Grid 1994; Ma and Liu 1994). A fitting optimization t-test chart analysis showed that the median DBH of dead Keram et al. Forest Ecosystems (2021) 8:49 Page 6 of 13 Fig. 2 Diameter class distribution of P. euphratica gap makers in the study area trees, especially those died before 1996, were larger than habitats. P. pruinosa, P. hendersonii, C. segetum, C. auri- that of living trees. However, by comparing DBH, we culatum, A. pungens, T. mongolicum, M. platyphylla and found that the trees that died in the 1997–2016 were L. ruthenicum were rare species, which only appeared in smaller than living trees. the RBAs. P. australis, H. halodendron and A. sparsifolia grew in the CGAs, but not in the UCAs or RBAs. Forest regeneration Additionally, the RBAs contained a high number of P. Riparian vegetation was distributed on both sides of the euphratica species (30) compared to other habitats, and riverbank and was relatively simple and sparse in struc- the number of P. euphratica and other species in the ture. In our field investigation, we counted 23 seedling CGAs was higher than in the UCAs. and sapling species belonging to 12 families and 20 The seedling and sapling density of P. euphratica was genera. In general, there were obvious differences in significantly higher in the RBAs than in the CGAs and seedling and sapling regeneration among the habitats UCAs. However, compared with the UCAs, the CGAs (CGAs, UCAs, and RBAs) (Table 3). Among the 14 plant promote tree regeneration to some extent by providing fa- species found in the CGAs, 7 were also found in the vorable conditions for the survival and growth of seedling UCAs and 19 were also found in the RBAs; P. euphra- and saplings (Table 4). We also performed quantitative tica, T. ramosissima, T. hispida, T. leptostachys, G. ura- analysis of seedling and sapling regeneration, based on lensis and E. oxycarpa commonly appeared in all three species diversity and richness (Fig. 5). According to Table 2 Analysis of population structure dynamics of gap makers under different volumes of runoff water 8 −3 Years Runoff (10 m ) V V V V V PV ′ P′ V ′′ 1 2 3 4 pi pi pi 1957–1966 33.34 −0.92 0.75 0.69 −0.67 − 0.073 0.0095 − 0.0040 0.2670 −0.0180 1967–1976 31.05 −0.77 0.55 0.25 0.00 −0.060 0.0040 −0.0019 0.1430 −0.0300 1977–1986 27.85 −0.84 0.78 0.71 −0.60 0.000 0.0080 0.0025 0.5000 0.0600 1987–1996 21.83 0.87 0.80 0.33 −0.33 0.040 0.0039 0.0071 0.6350 0.0970 1997–2006 22.84 0.00 0.40 1.00 −1.00 0.200 0.0133 0.0057 0.5600 0.0400 2007–2016 8.10 −0.57 0.57 0.13 0.33 0.460 0.0154 0.0084 0.6599 0.0554 V I–II dynamic index value of I–II class sizes, V II–III dynamic index value of II–III class sizes, V III–IV dynamic index value of III–IV class sizes, V IV–V dynamic 1 2 3 4 index value of IV–V class sizes, V dynamic index value under no external disturbance, V ′ dynamic index value under completely random disturbance, V ″ pi pi pi dynamic index value of non-random disturbance, P maximum risk value under completely random disturbance, P′ maximum risk value under non-random disturbance Keram et al. Forest Ecosystems (2021) 8:49 Page 7 of 13 Fig. 3 Relationship between population structure dynamics of gap makers and annual runoff analysis of the Margalef and Shannon-Wiener indices, the C. pseudophragmites, M. platyphylla, H. glomeratu and species diversity of seedlings and saplings was remarkably E. oxycarpa, as well as some herbs such as G. glabra, I. higher in the RBAs than in the other two areas, and higher salsoloides, K. caspica, L. ruthenicum, and A. sparsifolia, in the CGAs than in the UCAs (P < 0.05). Further, though although they were very rare. the density and diversity of seedlings and saplings in the CGAs was not as high as in the RBAs, the survival rate of Discussion young trees was higher in the CGAs than in the RBAs One or more gap makers, i.e., dead trees, create canopy (Table 4). gaps in forests (Zang et al.1999). Keram et al. (2019) found that the mortality of gap makers did not substan- Comparison of gap makers and gap fillers tially contribute to increased gap size in floodplain for- The statistical analysis showed that P. euphratica is the ests along the Tarim River. This could be explained by most common species of gap maker. However, it also the small crown and DBH of gap makers. Tree mortality existed as gap filler, along with T. ramosissima and P. is caused by different agents, including drought or flood- australis (Fig. 6). Other gap fillers included P. pruinosa, ing, strong wind, diseases, and insect pests, and affects A. venetum, H. halodendron, P. hendersonii, C. segetum, forest dynamics (Krasny and Whitmore 1992; Battles C. auriculatum, A. pungens, T. mongolicum, S. collina, and Fahey 2000). Host-specific disturbance was Fig. 4 Boxplots of the DBH of dead trees and living trees on the grid charts in canopy gaps that were created at different times. The box encloses the middle 80% of observations. Median and mean values are indicated by a vertical line (−) and a star (★), respectively. The upper whisker indicates the largest DBH value, while the lower whisker shows the smallest. The plus sign (+) indicates the maximum DBH Keram et al. Forest Ecosystems (2021) 8:49 Page 8 of 13 Table 3 Seedling and sapling recruitment in different habitats strong winds or insect pests may also play a role (Han in the study area et al. 2011). Species CGAs UCAs RBAs Water is the most important limiting factor for the maintenance and growth of P. euphratica in arid ecosys- P. euphratica 6330 tems, and P. euphratica depends on ground water that P. pruinosa –– 5 available to its roots (Yu et al. 2019). It is known that T. ramosissima 10 5 11 water is the factor with most influence on the survival, T. hispida 324 distribution, and development of vegetation in hyper- T. leptostachya 213 arid regions. In arid ecosystems, the vitality of P. euphra- P. australis 10 –– tica forests strongly depend on the groundwater level (Halik et al. 2006, 2009; Wang et al. 2008; Ginau et al. H. halodendron 2 –– 2013; Keyimu et al. 2017). Halik et al. (2019) reported G. uralensis 524 that the vitality of P. euphratica diminished when the A. sparsifolia 2 –– groundwater level was not sufficient for the development H. glomeratus –– 2 of tree stands. In addition, Xu et al. (2016) studied the K. caspica 3 – 4 correlation between groundwater and surface water I. salsoloides 1 – 2 using stable isotope technology. Their results indicated that the groundwater level is significantly affected by P. hendersonii –– 1 surface water. Chen et al. (2004) and Keilholz et al. C. segetum –– 2 (2015) also demonstrated that groundwater is often sup- C. auriculatum –– 3 plied by stream flow and infiltration from watercourses. A. pungens –– 2 The groundwater depth in the Tarim Basin has dropped S. collina 2 – 4 continuously as a result of the desiccation of streams C. pseudophragmites 4 – 5 and decreases in stream flow since an embankment was built in 2000. Thus, the physical characteristics of plant T. mongolicum –– 3 habitats have been changed. Consequently, the growth M. platyphylla –– 4 of P. euphratica cannot be sustained as before in this re- L. ruthenicum –– 2 gion, and forests have highly degraded or died. Accord- E. oxycarpa 113 ing to the present study, hydrological alterations may be A. venetum – 3 – the main cause of tree death in desert riparian forests CGAs Canopy gap areas, UCAs Undercanopygap areas, RBAs Riverbank areas along the Tarim River. DBH is one of the most common forest inventory variables and can reflect the population structure and growth status of tree stands (Condit et al. considered to be the most common agent of tree mortal- 2000; Di et al. 2014). Due to the unique climate and ity in old-growth mixed beech forests in the Western habitat conditions of desert floodplain areas, tree shape Carpathians (Orman and Dorota 2017), while wind dis- variables are obviously different from those in temperate turbance led to a disproportionate number of gap or/and subtropical forests (Ling et al. 2015). In the makers in old-growth red spruce–Fraser fir forests in present research, it was found that most young gap North Carolina (White et al. 1985) and red spruce–bal- makers (DBH < 15 cm) died in the most recent two de- sam stands in New Hampshire (Foster and Reiners 1986; cades, i.e., 1997–2006 and 2007–2016. In the early Worrall et al. 2005). However, the causes of tree mortal- 1970s, water resources were affected on a regional scale ity in riparian forests are different from those in other by climate change and anthropogenic activities (in- forest types, such as tropical and subtropical forests. The creased water use for irrigation), and then lag effects led results of the present study showed that tree mortality in to a progressive decrease in surface water from 1984 and riparian forests along the Tarim River was mainly influ- caused a reduction in groundwater from 2004 (Keram enced by hydrological factors (surface water), although et al. 2019). This resulted in habitat deterioration and a Table 4 Comparison of P. euphratica seedlings and saplings in different habitats in the middle reaches of the Tarim River Plot Occurrence Seedling and sapling Plant height (cm) Young tree density Plant height (cm) −2 −2 frequency (%) density (Plant∙m ) (Plant∙m ) CGAs 45.63 0.12 ± 0. 06b 3.53 ± 0.86b 8.31 ± 1.64a 285.0 ± 1.62a UCAs 34.52 0.08 ± 0.03b 3.12 ± 0.45b 0.28 ± 0.06b 167.0 ± 1.55b RBAs 100.00 17.56 ± 3.12a 18.76 ± 3.95a 0.34 ± 0.04b 187.0 ± 1.51b CGAs canopy gap areas, UCAs undercanopy areas, RBAs riverbank areas Keram et al. Forest Ecosystems (2021) 8:49 Page 9 of 13 Fig. 5 Species diversity of seedlings and saplings in different habitats decline in the survival of young P. euphratica individ- by the hydrological conditions, and only a small propor- uals. Based on quantitative correlation analysis, the tion of young trees survived to reach canopy height. present study revealed that gap maker mortality and However, there were a large number of larger, mature DBH structure responded negatively to variations in sur- trees among the living trees that have developed root face water. This may be due to long-lasting exogenous systems and are therefore not significantly affected by disturbances, such as a scarcity of surface water or a de- the reduced groundwater level. crease in groundwater level, having a notable influence By creating environmental heterogeneity in terms of on vegetation through drought stress. Thus, the water light availability, gap disturbances play a key role in requirement of riparian vegetation was not satisfied. forest regeneration as well as in the establishment and Therefore, gap makers of low DBH appeared frequently development of tree species with different ecological in P. euphratica forests, which experienced an increase recruitment patterns (Runkle 1989; Peterken 1996; in more resilient species. This may be because the vital- Mountford 2006; Zhang et al. 2006; Li et al. 2013). Gap- ity of P. euphratica in this arid area was directly affected based restoration provides a flexible system that Fig. 6 Comparison of gap maker and gap filler species in the middle reaches of the Tarim River Keram et al. Forest Ecosystems (2021) 8:49 Page 10 of 13 emulates natural regeneration under different environ- CGAs, and lowest in the UCAs. It may have been af- ments. Van and Dignan (2007) reported that seedling fected by light availability and hydrological conditions and sapling diversity and richness were promoted by during seedling and sapling recruitment. light intensity. Zhu et al. (2014) stated in a review of In conclusion, searching the tree mortality and plant published studies that compared to UCAs, the seedlings regeneration are considered to be the most effective and saplings of shade-tolerant species have significantly ways to clarify the development trend of desert riparian higher density in CGAs. Overall, water, light availability, forest. Furthermore, the protection and maintenance of and seedbed conditions potentially affect seedling emer- the floodplain vegetation along the Tarim River are cru- gence, germination, and establishment in floodplain ri- cial to sustainable regional development in the Tarim parian forests. It was found that appearance of seedling Basin. Our study area is located in the Yingbazha is closely related to flooding patterns in this study. Be- section, where there are fragment pits of stagnant water sides, a higher density of seedlings and saplings is ex- by flooding. Moreover, though the density and diversity pected in gaps as they provide full light conditions. The of seedlings and saplings in the CGAs was not as high as results of the present observations showed that the seed- in the RBAs, the survival rate of young trees was higher ling and sapling species diversity was higher in the CGAs in the CGAs than in the RBAs. Accordingly, we suggest than in the UCAs. Hence, forest canopy gaps playing a that making full use of limited resources of stagnant pivotal role in the long-term germination and regener- water pits to establish a “Natural Nursery of the Poplar” ation of plant species in some degree. Oliver and Larson and appropriately transplanted into the gap, the main (1996) stated that canopy gaps simultaneously result in purpose of which is to achieve an improved regeneration mortality in some individuals and establishment and efficiency of the Populus euphratica riparian forests. growth in others, and the ongoing process of death and Additionally, there is a need for further applied experi- replacement has a profound effect on forest structure ment to establish test areas for flood water allocation and composition. Pham et al. (2004) showed that Abies and utilization strategies, which, if successful, should balsamea is the most frequent successor in Abies forest then be extended to the whole basin. Thus, we argue stands and that Picea mariana is the most likely replace- that, to some degree, canopy gaps can provide a favor- ment species in Picea forest stands, regardless of the able environment for seedlings to grow into saplings, species of gap makers. In the present study, canopy gaps thereby reaffirming the importance of gap creation for were filled with shrubs and herbs. When we examined forest regeneration. the probability of replacement in floodplain forests, there Abbreviations seemed to be a reciprocal replacement of P. euphratica by DRB: Distance from river bank; GWL: Groundwater level; DBH: Diameter at T. ramosissima.and P. australis (Fig. 6). However, the the breast height; TH: Tree height; CGAs: Canopy gap areas; probability of P. euphratica replacement was lower when UCAs: Undercanopy areas; RBAs: Riverbank areas; THNR: Tarim Huyanglin nature reserve the mortality of young trees was particularly high. Acknowledgments Conclusion and suggestion Cordial thanks to MSc-Students (Elyar, Wang Wenjuan, Ramile, Mihray and As the largest riparian forest ecosystem in Northwest Gao Qing) from the Xinjiang University for their intensive support during the field investigation. Special thanks to Xinjiang LIDAR Applied Engineering China, Euphrates poplar forests are facing great chal- Technology Research Center for giving us free use of the Riegl VZ-1000 Ter- lenges of climate change and human interference (Zhang restrial Laser Scanner. We thank the anonymous reviewers for their construct- et al. 2016; Halik et al. 2019). Monitoring of tree mortal- ive comments that greatly helped us improve the quality of this manuscript. ity and plant regeneration in different habitats is consid- Authors’ contributions ered the most effective way of ensuring the sustainable Ayjamal Keram conceived, designed and performed the experiments, regeneration and ecological restoration of floodplain for- contributed the data collection, analyzed the data, prepared figures and ests along the Tarim River. Based on evaluation of the tables, and authored or approved the final draft of the manuscript. Ümüt Halik conceived and designed the experiments, contributed the data correlation between tree mortality and water resources collection, reviewed the manuscript, and approved the final draft. Tayierjiang in the study area, hydrological conditions (runoff vol- Aisahan, Maierdang Keyimu and Kadeliya Jiapaer participated in field work, ume) are the main reason for the death of P. euphratica contributed the data collection/validation and language proofreading. Guolei Li edited and validated the manuscript with critical comments and reviewed forests. At the beginning of the 1970s, hydrological con- the manuscript draft. All authors checked and approved the final manuscript. ditions were affected on a regional scale by anthropo- genic activities. Consequently, riparian forests were Funding threatened under water scarcity, which resulted in high The work was funded by the National Natural Science Foundation of China (31860134, U1703102, 31700386). mortality among Populus species. Our observations of seedling and sapling density indicated that the frequency Availability of data and materials of plant species was different among the three studied The data set generated for the study area is available from the habitats; it was highest in the RBAs, intermediate in the corresponding author on reasonable request. Keram et al. Forest Ecosystems (2021) 8:49 Page 11 of 13 Declarations Droessler L, Von LB (2005) Canopy gaps in two virgin beech forest reserves in Slovakia. J Sci 51(10):446–457. https://doi.org/10.17221/4578-JFS Ethics approval and consent to participate Foetzki A (2004) Transpiration and sap flow in Alhagi sparsifolia, Calligonum Not applicable. caput-medusae, Populus euphratica and Tamarix ramosissima. In: Runge M, Zhang X (eds) Ecophysiology and habitat requirements of perennial plant species in the Taklimakan Desert. Shaker Press, Aachen, pp 67–74 Consent for publication Foster JR, Reiners WA (1986) Size distribution and expansion of canopy gaps in a Not applicable. northern Appalachian spruce-fir forest. Vegetatio 68(2):109–114 Ginau A, Opp C, Sun Z, Halik Ü (2013) Influence of sediment, soil, and micro-relief Competing interests conditions on vitality of Populus euphratica stands in the lower Tarim riparian The authors declare that they have no competing interests. ecosystem. Quatern Intern 311:146–154. https://doi.org/10.1016/j.quaint.2013. 06.025 Author details Gries D, Zeng F, Foetzki A, Arndt SK, Bruelheide H, Thomas FM (2003) College of Resources and Environmental Science, Key Laboratory for Oasis Growth and water relations of Tamarix ramosissima and Populus Ecology of Ministry of Education, Xinjiang University, Ürümqi 830046, China. euphratica on Taklamakan desert dunes in relation to depth to a College of Forestry, Key Laboratory for Silviculture and Conservation of permanent water table. Plant Cell Environ 26(5):725–736. https://doi.org/1 Ministry of Education, Beijing Forestry University, Beijing 100083, China. 0.1046/j.1365-3040.2003.01009.x State Key Laboratory of Urban and Regional Ecology, Research Center for Halik Ü, Aishan T, Betz F, Kurban A, Rouzi A (2019) Effectiveness and challenges Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, of ecological engineering for desert riparian forest restoration along China’s China. largest inland river. Ecol Eng 127:11–22. https://doi.org/10.1016/j.ecoleng.201 8.11.004 Received: 26 February 2021 Accepted: 9 June 2021 Halik Ü, Chai Z, Kurban A, Cyffka B (2009) The positive response of some ecological indices of Populus euphratica to the emergency water transfer in the lower reaches of the Tarim River. Res Sci 31:1309–1314. https://doi.org/1 0.1007/978-3-642-00205-2_9 References Halik Ü, Kurban A, Mijit M, Schulz J, Parroth F, Coenradie B (2006) The potential Aishan T (2016) Degraded Tugai forests under rehabilitation in the Tarim riparian influence of embankment engineering and ecological water transfer on the ecosystem, Northwest China: monitoring, assessing and modeling. KU riparian vegetation along the middle and lower reaches of the Tarim River. Eichstätt-Ingolstadt, Germany In: Hoppe T, Kleinschmit B, Roberts B, Thevs N, Halik Ü (eds) Watershed and Aishan T, Betz F, Halik Ü, Cyffka B, Rouzi A (2018) Biomass carbon sequestration floodplain management along the Tarim River in China’s arid northwest. potential by riparian forest in the Tarim River watershed, Northwest China: Shaker Press, Aachen, pp 221–236 implication for the mitigation of climate change impact. Forests 9(4):196– Han L, Wang H, Chen J (2011) Disturbance regimes and gaps characteristics of 211. https://doi.org/10.3390/f9040196 the desert riparian forest at the middle reach of Tarim River. Acta Ecol Sin Albanesi E, Gugliotta OI, Mercurio L, Mercurio R (2005) Effects of gap size and 31(16):4699–4708. https://doi.org/10.1007/s11676-011-0113-8 within-gap position on seedlings establishment in silver fir stands. iForest Han WJ, He JF, Zhang WH, Li JX (2013) Effects of gap size on root growth of 1(1):55–59. https://doi.org/10.3832/ifor0448-0010055 seedlings and soil physical and chemical properties in plantation in the Battles JJ, Fahey TJ (2000) Gap dynamics following forest decline: a case study of Huanglong forest. Sci Silv Sin 11(49):17–23 red spruce forests. Ecol Appl 10(3):760–774. https://doi.org/10.2307/2641043 Hu S, Zhao R, Tian C, Song Y (2009) Empirical models of calculating phreatic Betz F, Halik Ü, Kuba M, Aishan T, Cyffka B (2015) Controls on aeolian sediment evaporation from bare soil in Tarim River basin, Xinjiang. Environ Geol 59(3): dynamics by natural riparian vegetation in the eastern Tarim Basin, Northwest 663–668. https://doi.org/10.1007/s12665-009-0063-z China. Aeolian Res 18:23–34. https://doi.org/10.1016/j.aeolia.2015.04.005 Huang P (2002) Irrigation-free vegetation and its recovery in arid region. Science Bottero A, Garbarino M, Dukić V, Govedar Z, Lingua E, Nagel TA (2011) Gap-phase Press, Beijing, pp 15–50 dynamics in the old-growth forest of Lom, Bosnia and Herzegovina. Silv Huang T, Pang Z (2011) Changes in groundwater induced by water diversion in Fenn 45(5):875–887. https://doi.org/10.14214/sf.76 the lower Tarim River, Xinjiang Uygur, NW China: evidence from Chen XD (1998) A study on the method of quantitative analysis for plant environmental isotopes and water chemistry. J Hydrol 387(3–4):188–201. population and community structural dynamics. Acta Ecol Sin 18(2):214–217 https://doi.org/10.1016/j.jhydrol.2010.04.007 Chen YJ, Li WH, Liu JZ, Yang YH (2013) Effects of water conveyance Huang X, Chen YN, Ma JX, Chen YP (2010) Study on change in value of embankments on riparian forest communities at the middle reaches of the ecosystem service function of Tarim River. Acta Ecol Sin 30(2):67–75. https:// Tarim River, Northwest China. Ecohydrology 6(6):937–948. https://doi.org/10.1 doi.org/10.1016/j.chnaes.2010.03.004 002/eco.1418 Jankovska I, Brūmelis G, Nikodemus O, Kasparinskis R, Amatniece V, Straupmanis Chen YN, Wang Q, Ruan X, Li WH, Chen YP (2004) Physiological response of G (2015) Tree species establishment in urban forest in relation to vegetation Populus euphratica to artificial water-recharge of the lower reaches of the composition, tree canopy gap area and soil factors. Forests 6(12):4451–4461. Tarim River. Acta Bot Sin 46(12):1393–1401 https://doi.org/10.3390/f6124379 China Green Foundation (2018) Ecological Restoration Plan of Populus euphratica Keilholz P, Disse M, Halik Ü (2015) Effects of land use and climate change on Forest Along the Belt and Road. China Forestry Publishing House, Beijing groundwater and ecosystems at the middle reaches of the Tarim River using Coates KD, Philip JB (1997) A gap-based approach for development of the MIKE SHE integrated hydrological model. Water 7(12):3040–3056. https:// silvicultural systems to address ecosystem management objectives. Forest doi.org/10.3390/w7063040 Ecol Manag 99(3):337–354. https://doi.org/10.1016/S0378-1127(97)00113-8 Keram A, Halik Ü, Keyimu M, Aishan T, Mamat Z, Rouzi A (2019) Gap dynamics of Condit R, Ashton PS, Baker P, Bunyavejchewin S (2000) Spatial patterns in the natural Populus euphratica floodplain forests affected by hydrological distribution of tropical tree species. Science 288(5470):1414–1418. https://doi. alteration along the Tarim River: implications for restoration of the riparian forests. org/10.1126/science.288.5470.1414 Forest Ecol Manag 438:103–113. https://doi.org/10.1016/j.foreco.2019.02.009 Deng MJ (2009) Theory and practice of water management in the Tarim River, Kern CC, Burton JI, Raymond P, D'Amato AW, Keeton WS (2017) Challenges China. Science Press, Beijing facing gap-based silviculture and possible solutions for Mesic northern Deng MJ (2016) Prospecting development of southern Xinjiang: water strategy forests in North America. Forestry 90(1):4–17. https://doi.org/10.1093/forestry/ and problem of Tarim River basin. Arid Land Geog 39(1):1–11 cpw024 Di K, Guo YX, Ren CJ, Zhao FZ, Feng YZ, Han XH, Yang GH (2014) Population Kern CC, Montgomery RA, Reich PB, Strong TF (2013) Canopy gap size influences structure and spatial pattern of main tree species in secondary Betula niche partitioning of the ground-layer plant community in a northern platyphylla forest in Ziwuling Mountains, China. Sci Rep 4(1):6873. https://doi. temperate forest. J Plant Ecol 6(1):101–112. https://doi.org/10.1093/jpe/rts016 org/10.1038/srep06873 Dorotä D, Thomast V (2008) Tree fall-gap structure and regeneration in mixed Keyimu M, Halik Ü, Rouzi A (2017) Relating water use to tree vitality of Populus Abies alba stands in Central Poland. Forest Ecol Manag 255(8):3469–3476. euphratica Oliv. In the lower Tarim River, NW China. Water 8(9):622. https:// https://doi.org/10.1016/j.foreco.2008.02.025 doi.org/10.3390/w9080622 Keram et al. Forest Ecosystems (2021) 8:49 Page 12 of 13 Keyimu M, Halik Ü, Wumaier G, Abliz A (2018) Diel variation of Populus euphratica Rentch JS, Schuler TM, Nowacki GJ, Beane NR, Ford WM (2010) Canopy gap sap flow and its correlation with meteorological factors. J Glaciol Geocryol dynamics of second-growth red spruce-northern hardwood stands in West 40(1):166–175 Virginia. Forest Ecol Manag 260(10):1921–1929. https://doi.org/10.1016/j. Kitao M, Kitaoka S, Harayama H (2018) Canopy nitrogen distribution is optimized foreco.2010.08.043 to prevent photo inhibition throughout the canopy during sun flecks. Sci Rouzi A, Halik Ü, Thevs N, Aishan T (2018) Water efficient alternative crops for Rep 8(1):503–513. https://doi.org/10.1038/s41598-017-18766-0 sustainable agriculture along the Tarim Basin: a comparison of the economic potentials of Apocynum pictum, Chinese red date and cotton in Xinjiang, Krasny ME, Whitmore MC (1992) Gradual and sudden forest canopy gaps in China. Sustainability 10(1):1–17. https://doi.org/10.3390/su10010035 Allegheny northern hardwood forests. Can J Forest Res 22(2):139–142. https://doi.org/10.1139/x92-019 Rumbaur C, Thevs N, Disse M (2015) Sustainable management of river oases Lang P, Ahlborn J, Schäfer P, Wommelsdorf T, Jeschke M, Zhang X, Thomas FM along the Tarim River (SuMaRiO) in northwestern China under conditions of (2016) Growth and water use of Populus euphratica trees and stands with climate change. Earth Sys Dynam 5(2):1221–1273. https://doi.org/10.5194/ different water supply along the Tarim River, Northwest China. Forest Ecol esd-6-83-2015 Manag 380:139–148. https://doi.org/10.1016/j.foreco.2016.08.049 Runkle JR (1989) Synchrony of regeneration, gaps, and latitudinal differences in Li ZP, Li WM, Wu FZ, Yang WQ, Xu ZF, Tan P, Wu QG (2013) Effects of forest gap tree species diversity. Ecology 70(3):546–547. https://doi.org/10.2307/1940199 on content of soil nit rate nitrogen and ammonium nitrogen at different Runkle JR (1998) Changes in southern Appalachian canopy tree gaps sampled critical periods of subalpine forest in Western Sichuan. J Soil Water Conserv thrice. Ecology 79(5):1768–1780. https://doi.org/10.2307/176795 6(27):270–274 Runkle JR, Yetter TC (1987) Treefalls revisited: gap dynamics in the southern Ling H, Zhang P, Xu H, Zhao X (2015) How to regenerate and protect desert Appalachians. Ecology 68(2):417–424. https://doi.org/10.2307/1939273 riparian Populus euphratica forest in arid areas. Sci Rep 5(154):81–89. https:// Schliemann SA, Bockheim JG (2011) Methods for studying tree fall gaps: a doi.org/10.1038/srep15418 review. Forest Ecol Manag 261(7):1143–1151. https://doi.org/10.1016/j. Liu Q, Hytteborn H (1991) Gap structure, disturbance and regeneration in a foreco.2011.01.011 primeval Picea abies forest. J Veg Sci 2(3):391–402. https://doi.org/10.23 Sharma LN, Shrestha KB, Maren IE (2018) Tree regeneration in gap-understory 07/3235932 mosaics in a subtropical shorea robusta (Sal) forest. J Forest Res 30(6):2061– Lu DL, Wang GG, Yan QL, Gao T, Zhu JJ (2018) Effects of gap size and within-gap 2068. https://doi.org/10.1007/s11676-018-0747-x position on seedling growth and biomass allocation: is the gap partitioning Song YD, Fan ZL, Lei ZD, Zhang FW (2000) Research on water resources and hypothesis applicable to the temperate secondary forest ecosystems in ecology of Tarim River, China. Xinjiang People’s Press, China, Beijing Northeast China? Forest Ecol Manag 429:351–362. https://doi.org/10.1016/j. Thevs N (2006) Ecology, spatial distribution, and utilization of the Tugai foreco.2018.07.031 vegetation at the middle reaches of the Tarim River, Xinjiang, China. Ernst- Ma KP (1994) Measurement of biotic community diversity: I. a diversity (part. 1). Moritz-Arndt University of Greifswald, Germany Biodivers Sci 2(3):162–168. https://doi.org/10.17520/biods.1994027 Thevs N, Zerbe S, Schnittler M, Abdusalih N, Succow M (2008) Structure, Ma KP, Liu YM (1994) Measurement of biotic community diversity: I. a reproduction and flood-induced dynamics of riparian Tugai forests at the diversity (part. 2). Biodive Sci 2(4):231–239. https://doi.org/10.17520/ Tarim River in Xinjiang, NW China. Forestry 81(1):45–57. https://doi.org/10.1 biods.1994027 093/forestry/cpm043 Mamat Z, Halik Ü, Aishan T, Aini A (2019) Ecological effect of the riparian Thomas AN, Jurij D (2006) Intermediate wind disturbance in an old-growth ecosystem in the lower reaches of the Tarim River in Northwest China. Plos beech-fir forest in south eastern Slovenia. Can J For Res 36(3):629–638. One 14(1):e0208462. https://doi.org/10.1371/journal.pone.0208462 https://doi.org/10.1139/x05-263 Mamat Z, Halik Ü, Keyimu M, Keram A, Nurmamat K (2018) Variation of the Thomas FM, Lang P (2020) Growth and water relations of riparian poplar forests floodplain forest ecosystem service value in the lower reaches of the Tarim under pressure in Central Asia's Tarim River basin. River Res Applic 37(2):233– River, China. Land Degrad Dev 29(1):47–57. https://doi.org/10.1002/ldr.2835 248. https://doi.org/10.1002/rra.3605 Mountford EP (2006) Patterns of regeneration and ground vegetation associated Thomas FM, Yu R, Schäfer P, Zhang XM, Petra L (2017) How diverse are Populus with canopy gaps in a managed beechwood in southern England. Forestry “diversifolia” leaves? Linking leaf morphology to eco-physiological and stand 79(4):389–408. https://doi.org/10.1093/forestry/cpl024 variables along water supply and salinity gradients. Flora 233:68–78. https:// Nagel TA, Svoboda M (2008) Gap disturbance regime in an old-growth Fagus doi.org/10.1016/j.flora.2017.05.007 abies forest in the Dinaric Mountains, Bosnia-Herzegovina. Can J For Res Van PJD, Dignan P (2007) Regeneration after 8 years in artificial canopy gaps in 38(11):2728–2737. https://doi.org/10.1139/X08-110 mountain ash (Eucalyptus regnans) forest in south-eastern Australia. Forest Nagel TA, Svoboda M, Kobal M (2016) Disturbance, life history traits, and Ecol Manag 244(1):102–111. https://doi.org/10.1016/j.foreco.2007.03.055 dynamics in an old-growth forest landscape of southeastern Europe. Ecol Walker LR (2000) Seedling and sapling dynamics of treefall pits in Puerto Rico1. Appl 24(4):663–679. https://doi.org/10.1890/13-0632.1 Biotropica 32(2):262–275. https://doi.org/10.1111/j.1744-7429.2000.tb00469.x Nagel TA, Svoboda M, Rugani T, Diaci J (2010) Gap regeneration and Wang JS, Halik Ü, Cyffka B, Kurban O, Peng SH (2008) Study on DBH-structure of replacement patterns in an old-growth Fagus-Abies forest of Bosnia- Populus euphratica and their spatial distribution in the lower reaches of the Herzegovina. Plant Ecol 208(2):307–318. https://doi.org/10.1007/s11258-009- Tarim River. Chin Bull Bot 25(6):728–733. https://doi.org/10.3969/j.issn.1674-34 9707-z 66.2008.06.014 Oliver CD, Larson BC (1996) Forest stand dynamics. Willey, New York Wang SJ, Chen BH, Li HQ (1995) Euphrates poplar forest, 1st end. China Environ Orman O, Dorota D (2017) Gap dynamics in the Western Carpathian mixed Sci Press, Beijing beech old growth forests affected by spruce bark beetle outbreak. Eur J Wen K (2016) Study on the relationship among Abies faxoniana regeneration, Forest Res 136(4):571–581. https://doi.org/10.1007/s10342-017-1054-3 canopy gaps, and Fargesia denudate in subalpine coniferous forest in Peterken GF (1996) Natural woodland: ecology and conservation in northern southwestern Sichuan, China. Beijing Forestry University, Beijing temperate regions (2001 reprint). J Ecol 84(5):790. https://doi.org/10.2307/22 White PS, Mackenzie MD, Busing RT (1985) Natural disturbance and gap phase dynamics in southern Appalachian spruce-fir forests. Can J For Res 15(1):233– Petritan AM, Nuske RS, Petritan IC (2013) Gap disturbance patterns in an old- 240. https://doi.org/10.1139/x85-041 growth sessile oak (Quercus petraea L.), European beech (Fagus sylvatica L.) Worrall JJ, Lee TD, Harrington TC (2005) Forest dynamics and agents that initiate forest remnant in the Carpathian Mountains, Romania. Forest Ecol Manag and expand canopy gaps in Picea-abies forests of Crawford notch, New 308(4):67–75. https://doi.org/10.1016/j.foreco.2013.07.045 Hampshire, USA. J Ecol 93(1):178–190. https://doi.org/10.1111/j.1365-2745.2 Pham AT, Grandpré LD, Gauthier S, Bergeron Y (2004) Gap dynamics and 004.00937.x replacement patterns in gaps of the northeastern boreal forest of Quebec. Xu C, Chen Y, Li W (2006) Climate change and hydrologic process response in Can J For Res 34(2):353–364. https://doi.org/10.1139/x03-265 the Tarim River basin over the past 50 years. Chin Sci Bull 51(1):25–36. Popa I, Nechita C, Hofgaard A (2017) Stand structure, recruitment and growth https://doi.org/10.1007/s11434-006-8204-1 dynamics in mixed subalpine spruce and Swiss stone pine forests in the Xu MC, Chen YJ, Liu JZ, Wang D, Liu YQ (2016) The distribution pattern dynamics eastern Carpathians. Sci Total Environ 598:1050–1057. https://doi.org/10.1016/ of Populus euphratica are influenced by hydrology in the middle reaches of j.scitotenv.2017.04.169 Tarim River. Acta Ecol Sin 36(9):2646–2655 Keram et al. Forest Ecosystems (2021) 8:49 Page 13 of 13 Yamamoto SI (2000) Forest gap dynamics and tree regeneration. J Forest Res 5(4):223–230. https://doi.org/10.1007/BF02767114 Yang H, Liu SR, Cao KF, Wang JX, Li YD, Xu H (2017) Characteristics of typhoon disturbed gaps in an old-growth tropical montane rainforest in Hainan Island, China. J Forest Res 28(6):1231–1239. https://doi.org/10.1007/s11676-01 7-0402-y Yu HY, Ya NC, We HL (2009) Relationship between soil properties and plant diversity in a desert riparian forest in the lower reaches of the Tarim River, Xinjiang, China. Arid Soil Res Rehab 23(4):283–296. https://doi.org/10.1080/1 Yu TF, Feng Q, Si JH, Xi HY, O'Grady AP, Pinkard EA (2019) Responses of riparian forests to flood irrigation in the hyper-arid zone of NW China. Sci Total Environ 648:1421–1430. https://doi.org/10.1016/j.scitotenv.2018.08.287 Zang RG, Liu JY, Dong DF (1999) Gap dynamics and forest biodiversity. China Forestry Publishing House, Beijing, pp 1–242 Zeng F, Zhang X, Foetzki A, LiX RM (2002) Study on the water physiological characteristics of Populus euphratica in the Cele oasis. Arid Zone Res 19(2): 26–30 Zeng FJ, Bleby TM, Landman PA (2006) Water and nutrient dynamics in surface roots and soils are not modified by short-term flooding of phreatophytic plants in a hyperarid desert? Plant Soil 279(1–2):129–139. https://doi.org/10.1 007/s11104-005-0498-2 Zeng FJ, Zhang WJ, Liu GJ, Zhang DY, Li XY, Zhang L, Yuan LM, Zhang XM (2020) Stable restoration pattern and sustainable management technology of main dominant vegetation in typical desert areas of China. Bull Chin Acad Sci 214(6):63–70 Zhang CY, Zhao XH, Zheng JM (2006) A study on soil properties in forest gaps and under canopy in broad-leaved Pinus koraiensis forest in Changbai Mountain. Forest Res 19(3):347–352 Zhang Q, Gu XH, Singh VP (2016) Magnitude, frequency and timing of floods in the Tarim River basin, China: changes, causes and implications. Glob Planet Chang 139:44–55. https://doi.org/10.1016/j.gloplacha.2015.10.005 Zhao ZY, Zhang K, Lu L (2011) Seedling recruitment in desert riparian forest following river flooding in the middle reaches of the Tarim River. Acta Ecol Sin 31(12):3322–3329. https://doi.org/10.3724/SP.J.1011.2011.00338 Zhu JJ, Lu DL, Zhang WD (2014) Effects of gaps on regeneration of woody plants: a meta-analysis. J Forest Res 25(3):501–510. https://doi.org/10.1007/ s11676-014-0489-3 Zhu JJ, Mao ZH, Hu LL, Zhang JX (2007) Plant diversity of secondary forests in response to anthropogenic disturbance levels in montane regions of northeastern China. J Forest Res 12(6):403–416. https://doi.org/10.1007/s1031 0-007-0033-9 Zhu JJ, Matsuzaki T, Li FQ (2003) Effect of gap size created by thinning on seedling emergency, survival and establishment in a coastal pine forest. Forest Ecol Manag 182(1–3):339–354. https://doi.org/10.1016/S0378-1127(03 )00094-X Zhu JJ, Yan Q, Yu LZ, Zhang J, Yang K, Gao T (2018) Support ecological restoration and sustainable management of forests in Northeast China based on research of forest ecology and demonstrations. Bull Chin Acad Sci 33(1): 107–118 Zhu JJ, Zhang GQ, Wang GG (2015) On the size of forest gaps: can their lower and upper limits be objectively defined. Agric For Meteorol 213(2):64–76. https://doi.org/10.1016/j.agrformet.2015.06.015 Zhu JJ, Zhu CY, Lu DL, Wang GG, Zheng X, Cao JS, Zhang JX (2021) Regeneration and succession: a 50-year gap dynamic in temperate secondary forests, Northeast China. Forest Ecol Manag 484:118943. https://doi.org/10.1016/j. foreco.2021.118943 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Forest Ecosystems" Springer Journals

Tree mortality and regeneration of Euphrates poplar riparian forests along the Tarim River, Northwest China

Download PDF
13 pages

Loading next page...
 
/lp/springer-journals/tree-mortality-and-regeneration-of-euphrates-poplar-riparian-forests-KcpAYbWP7C

References (111)

Publisher
Springer Journals
Copyright
Copyright © The Author(s) 2021
eISSN
2197-5620
DOI
10.1186/s40663-021-00323-x
Publisher site
See Article on Publisher Site

Abstract

Background: Tree mortality and regeneration (seedling and sapling recruitment) are essential components of forest dynamics in arid regions, especially where subjected to serious eco-hydrological problems. In recent decades, the mortality of the Euphrates poplar (Populus euphratica) along the Tarim River in Northwest China has increased. However, few studies have quantified the causes of mortality and regeneration in this azonal riparian forest type. Methods: The present study describes the annual hydrological response of tree mortality and regeneration in forest gaps. A total of 60 canopy gaps were investigated in six replicate grid plots (50 m × 50 m) and the annual runoff and water consumption data during the period of 1955–2016 were collected from hydrological stations in the middle reaches of the Tarim River. We compared the regeneration density of seedlings and saplings within the canopy gap areas (CGAs), undercanopy areas (UCAs), and uncovered riverbank areas (RBAs) through detailed field investigation. Results: Our study found that the mortality of young and middle-aged gap makers has increased remarkably over recent decades, particularly since the year 1996. The main results indicated that regional water scarcity was the primary limiting factor for long-term changes in tree mortality, as shown by a significant correlation between the diameter at breast height (DBH) of dead trees and the annual surface water. The average density (or regeneration rate) of seedlings and saplings was highest in the RBAs, intermediate in the CGAs, and lowest in the UCAs. Compared with the UCAs, the CGAs promote tree regeneration to some extent by providing favorable conditions for the survival and growth of seedlings and saplings, which would otherwise be suppressed in the understory. Furthermore, although the density of seedlings and saplings in theCGAswas notashighasinthe RBAs, thesurvivalratewas higher in the CGAsthaninthe RBAs. Conclusion: Forest canopy gaps in floodplain areas can play a decisive role in the long-term germination and regeneration of plant species. However, as a typical phreatophyte in this hyper-arid region, the ecosystem structure, functions and services of this fragile P. euphratica floodplain forests are threatened by a continuous decrease of water resources, due to excessive water use for agricultural irrigation, which has resulted in a severe reduction of intact poplar forests. Furthermore, the survival of seedlings and saplings is influenced by light availability and soil water at the regional scale. Our findings suggest that policymakers may need to reconsider the restoration and regeneration measures implemented in riparian P. euphratica forests to improve flood water efficiency and create canopy gaps. Our results provide with valuable reference information for the conservation and sustainable development of floodplain forest ecosystems. Keywords: Tree mortality, Regeneration strategy, Seedling and sapling recruitment, Gap makers, Riparian forest, Tarim River * Correspondence: halik@xju.edu.cn College of Resources and Environmental Science, Key Laboratory for Oasis Ecology of Ministry of Education, Xinjiang University, Ürümqi 830046, China Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Keram et al. Forest Ecosystems (2021) 8:49 Page 2 of 13 Highlights ecosystem balance (Wang et al. 1995; Huang 2002; Huang and Pang 2011; Mamat et al. 2019). Nevertheless, Effect of hydrological alterations on tree mortality of the Tarim floodplain ecosystem, especially the riparian Populus euphratica riparian forest was quantified. vegetation, has been seriously degraded, as the water The contribution of different habitats (canopy gap conveyance in the main channel has rapidly decreased area, undercanopy area and riverbank area) on plant and many tributaries of the watershed have been discon- regenerations was evaluated. nected from the main river (Zhao et al. 2011; Chen et al. Recommendations for regeneration and succession 2013). of riparian forests at the Tarim River were put Many studies have demonstrated that water shortages forward. in the Tarim are mainly caused by climate change coupled with intensive anthropogenic activities, i.e., tree Introduction felling, overgrazing, excessive land reclamation and Riparian forests provide fundamental ecosystem services disorderly expansion of cotton monoculture. Therefore, in arid regions, including maintenance of ecological sta- restoring degraded floodplain ecosystems has become a bility and prevention of natural disasters such as sand- major focus of applied forest research (Deng 2009;Rum- storms, heatwaves, and desertification (Song et al. 2000; baur et al. 2015; Thomas et al. 2017; Thomas and Lang Chen et al. 2013; Betz et al. 2015; Mamat et al. 2018; 2020). Among all processes of natural forest dynamics, Halik et al. 2019). The existence of trees, bushes, and the formation of forest canopy gaps was considered a grass vegetation on the banks of the Tarim River, which vital regeneration strategy for maintaining floodplain constitute the natural barriers of river oases, is abso- forest structures (Han et al. 2011; Keram et al. 2019). lutely dependent on the river (Zeng et al. 2002, 2006; Gap disturbance is the main driving force of forest dy- Halik et al. 2006; Aishan et al. 2018). In recent decades, namics as it creates environmental heterogeneity (Zhu floodplain vegetation has been threatened by increased et al. 2003, 2007; Albanesi et al. 2005; Bottero et al. water scarcity, and large areas of riparian forests includ- 2011). Forest gaps thus create important habitat for the ing seedlings and saplings, have withered (Gries et al. regeneration of plant species, which may otherwise be 2003; Foetzki 2004; Aishan 2016; Zeng et al. 2020). With suppressed by the undercanopy (Han et al. 2011; Sharma China’s rapid economic development and the implemen- et al. 2018). They also play a vital role in forest regener- tation of the “Ecological Civilization” and “One Belt One ation and succession, especially in the establishment and Road” initiatives, the ecological restoration of the flood- development of plant species that differ in ecological re- plain ecosystem along the Tarim River has been desig- cruitment (Runkle 1998; Mountford 2006; Rentch et al. nated as a high priority by the Chinese government 2010; Han et al. 2013; Zhu et al. 2014; Jankovska et al. (Halik et al. 2006, 2019). Conservation of the remaining 2015). Nagel et al. (2010) reported a high presence of ad- riparian forests and recovery of the degraded parts of vanced regeneration in a gap regeneration study of a the ecosystem are crucial for further sustainable devel- (mixed) beech virgin forests. Besides, it also has been re- opment of the region (Rumbaur et al. 2015; Deng 2016; ported that canopy gaps close when the height of regen- China Green Foundation 2018), particularly, transport erations reached 20 m, consistent with the definition infrastructure, including railways, highways, and oil and used by Nagel and Svoboda (2008). Zhu et al. (2021) gas pipelines in the Tarim Basin are in need to be pro- found that with the decreased of gap size, pioneer spe- tected (Deng 2009, 2016; Aishan 2016; Halik et al. 2019). cies became the sub-canopy layer, and with the aging of Thus, it is necessary to preserve the functions and ser- gap, the light conditions changed over time, which was vices of these forests through adaptive management. conductive to the recruitment of shade-tolerant species; The riparian forest (also known as Tugai forests) at Keram et al. (2019) also revealed that hydrological con- the Tarim River constitute a natural green belt at the ditions (groundwater, runoff and water consumption) northern edge of the Taklimakan Desert. The Euphrates are the main driving force of the gap-scale disturbance poplar (Populus euphratica Oliv.) is a dominant tree spe- of desert riparian forests along the Tarim River. In flood- cies in the floodplain ecosystems. As a “Green Corridor”, plain forests, canopy gaps may not be filled with regen- P. euphratica forests have become increasingly import- erations within a short period because of high tree ant for preventing the unification of two neighboring mortality. This leads to the continual expansion of can- sandy deserts, the Taklimakan and the Kuruktagh (Xu opy gaps (Keram et al. 2019). The majority of earlier et al. 2006; Betz et al. 2015). These forests have import- studies have focused on the mortality of gap makers and ant ecological functions in addition to socio-economic the diversity of regeneration species in other various cli- and touristic value, such as protection of biodiversity, matic zones, such as tropical, subtropical, north temper- regulation of the climate and hydrologic conditions in ate, and cold temperate regions (Runkle and Yetter oases, fertilization of soils, and maintenance of regional 1987; Yamamoto 2000; Dorotä and Thomast 2008; Keram et al. Forest Ecosystems (2021) 8:49 Page 3 of 13 Petritan et al. 2013; Popa et al. 2017; Zhu et al. 2018; (H3) Kitao et al. 2018). However, systematic research on tree In floodplain forests, due to the degradation of plant mortality and plant regeneration of desert riparian for- habitat, canopy gaps may not be filled up with other re- ests under various hydrological scenarios are relatively generations in a relatively short period. By comparison lacking. Therefore, it is necessary to scientifically under- of the species diversity between gap makers and gap stand the response of P. euphratica mortality rate to fillers in canopy gaps, we hypothesized that canopy gaps hydrological dynamics at regional scales, and seedling may not be filled up with other regenerations within a and sapling regeneration in the Tarim River Basin. short time, (i) which may be due to the degraded habitat Forest canopy gaps may provide opportunities for of plant growth, (ii) prolonged and unexpected flooding species regeneration and have therefore been widely disturbance resulted in the degradation of P. euphratica exploited in forest recovery programs (Coates and Philip trees, and (iii) its mortality did not cause significant 1997; Schliemann and Bockheim 2011; Kern et al. 2013, changes in tree distribution but caused changes at tree 2017; Nagel et al. 2016; Lu et al. 2018). Thus, it is neces- level. sary to conduct studies to better understand the mortal- We intend to verification of these hypotheses by com- ity of gap makers under long-term hydrological piling data from six permanent monitoring plots located processes and seedling and sapling establishment in dif- at undisturbed natural forest sites along the middle ferent habitats, such as canopy gap areas (CGAs), under- reaches of the Tarim River. We believe that addressing canopy areas (UCAs), and riverbank areas (RBAs) of these hypotheses will contribute to improve the under- floodplain, as well as to clarify the contribution of can- standing of the current state of the Tarim riparian eco- opy gaps to plant regeneration in riparian forests. Such system, where P. euphratica is now in urgent need of information may provide insights into the efficiency of protection. Additionally, it will provide an ecological gap usage and seedling planting in forest restoration ac- framework for the design and implementation of close- tivities along the Tarim River. In the present study, we to-nature restoration techniques to extend P. euphratica used comprehensive field investigation data to describe forests. tree mortality as well as to compare the density of seed- lings and saplings between the CGAs, UCAs, and RBAs Methods to evaluate whether the CGAs are important for the re- Study site generation of P. euphratica riparian forests along the Field work was conducted in Yingbazha village (41.22°N, Tarim River. In this work we made three hypotheses: 84.31°E, ASL 1000 m) in Tarim Huyanglin Nature Reserve (THNR) along the middle reaches of the Tarim River (Fig. 1), which was established in 1984 and (H1) upgraded to a National Nature Reserve in 2006. The re- Based on the observation that canopy gap disturbances gion is an extremely arid warm temperate zone, with an have frequently emerged in the desert riparian forests annual mean precipitation of 75 mm and an annual and its composition and structure were drastically differ- mean temperature of 11.05 °C (Keyimu et al. 2018). The ent from other forest types (Han et al. 2011; Keram et al. annual potential evapotranspiration ranges from 2500 to 2019). We hypothesized that characteristics of tree mor- 3500 mm (Rouzi et al. 2018). According to the USDA tality (number, DBH) strongly responded to hydrological (United States Department of Agriculture) soil classifica- dynamics (such as stream flow) at local and regional tion system, the soil of the Tarim River is a member of scales in the Tarim River Basin, resulting in the emer- the Aridisol order, and the soil is silty loam (Hu et al. gence of special forest gap structures. 2009; Yu et al. 2009). The local groundwater system is recharged from the surface water through bank infiltration (Huang et al. 2010). The forest structure and composition (H2) in the study area is relatively simple, and the local flora is Conclusions on the size development of canopy gaps, mainly composed of P. euphratica, Tamarix ramosissima once formed they would continuously expand (Keram and Phragmites australis (Table 1). However, a few rare et al. 2019). The important process of gap dynamics, plant species are also present, including Populus pruinosa, i.e. plant regeneration, in floodplain forests has not Tamarix hispida, Tamarix leptostachys, Glycyrrhiza glabra, been quantified yet. To test the general hypothesis, Inula salsoloides, Karelinia caspica, Lycium ruthenicum, that canopy gaps promote plant regeneration in ripar- Alhagi sparsifolia, Apocynum venetum, Halimodendron ian forests, we formulated the hypothesis that canopy halodendron, Poacynum hendersonii, Cirsium segetum, gaps drive tree regeneration to some degree, due to Cynanchum auriculatum, Aeluropus pungens, Taraxacum canopy gap provides more adequate light conditions mongolicum, Salsola collina, Calamagrostis pseudophrag- than the UCAs. mites, Myricaria platyphylla, Halogeton glomeratus,and Keram et al. Forest Ecosystems (2021) 8:49 Page 4 of 13 Fig. 1 Sketch map of the study area Elaeagnus oxycarpa (Gries et al. 2003;Chenetal. 2004; were recorded for each tree. Morphological characteris- Thevs 2006;Thevs et al. 2008;Huang et al. 2010;Lang tics were used to determine the degree of tree decom- et al. 2016; Halik et al. 2019). position and the life expectancy of gap makers was estimated under the guidance of local experienced forest Field investigation workers (Liu and Hytteborn 1991; Droessler and Von A complete field survey was carried out from May 15 to 2005; Thomas and Jurij 2006; Petritan et al. 2013; Wen June 20, 2017. A total of 60 canopy gaps were investi- 2016; Yang et al. 2017; Keram et al. 2019), and the rela- gated in six replicate grid plots (50 m × 50 m) (Table 1). tive ages of the gap makers were thus determined. In Canopy gaps were created by tree mortality (i.e., death) addition, we determined whether there were significant or by > 50% loss of tree branches. The ratio of gap diam- differences in the DBH distribution among living and eter to tree height on the border of the gap (R value) dead trees. We regarded trees bordering canopy gaps as D/H ranged from 0.4 to 2.2 (Keram et al. 2019), which is con- living trees and gap makers as dead trees. To investigate sistent with measurements reported by Zhu et al. (2015). regeneration species, we selected five replicate plots in For each canopy gap, all surrounding trees and gap each habitat (CGAs, UCAs, and RBAs) and recorded the makers were identified, and the species, diameter at species and numbers of seedlings and saplings. We sim- breast height (DBH), and tree height (TH) or log length ultaneously compared the species diversity and density Table 1 Characteristics of six natural stands sampled for analysis of tree mortality Plots Area DRB GWL Lat. Long. Altitude P. euphratica T. ramosissima Gap (m ) (m) (m) (N) (°) (E) (°) (m) openness Ave. DBH (cm) Ave. TH (m) Ave. DBH (cm) Ave. TH (m) (%) 1 50 × 50 100 2.00 41.175 84.240 929 35.4 9.8 1.6 3.2 43.10 2 50 × 50 150 2.68 41.229 84.333 920 21.6 12.1 4.2 2.6 66.16 3 50 × 50 300 3.41 41.176 84.241 920 15.5 9.1 2.3 1.6 50.54 4 50 × 50 600 4.55 41.174 84.237 916 23.5 11.0 3.2 2.4 41.38 5 50 × 50 1200 5.45 41.252 84.340 915 28.4 10.5 5.4 2.2 52.41 6 50 × 50 2400 7.50 41.228 84.334 916 21.1 12.1 2.6 1.7 52.12 DRB Distance from river bank, GWL Groundwater level, DBH Diameter at the breast height, TH Tree height Keram et al. Forest Ecosystems (2021) 8:49 Page 5 of 13 in these plots. We defined a CGA as an area where the method was used to statistically analyze the change canopy gap > 10 m . For the UCA, we chose plots trend of gap makers that died in different years and to around the canopy gaps in the study area; RBA plots evaluate the distribution patterns of the DBH values of were located near the riverbank, which were covered gap makers. We used a chi-square goodness-of-fit test to with excessive water. For each habitat, 15 repeated sam- reveal the DBH distribution of dead trees versus living pling plots (10 m × 10 m) were set, giving a total of 45 trees. Further, we comparatively analyzed plant regener- sampling plots. In each plot, we counted the number of ation density between the different habitats (CGAs, species, for seedlings (≤10 cm tall) and saplings (10–100 UCAs, and RBAs) using the Margalef and Shannon- cm tall) (Walker 2000). Wiener indices. Values of P < 0.05 were considered to be statistically significant. The data were organized using Data processing Excel 2015 (Microsoft, Redmond, USA) and analyzed Classifying the DBH structure of gap makers using OriginPro 2016 (OriginLab Corp2016). Over the entire area of canopy gaps, gap makers were classified into five categories according to their DBH Results and TH (Xu et al. 2016): Class I comprised juvenile trees Tree mortality (DBH < 5 cm, TH < 4 m), Class II comprised young trees Among the 245 gap makers identified, the mean DBH (5 cm < DBH < 10 cm, TH ≥ 4 m), Class III comprised was 20.80 ± 9.03 cm (range 10.10–49.50 cm) and there young/middle-aged trees (10 cm < DBH < 15 cm), Class were significant variations in different years. The chi- IV comprised middle-aged trees (15 cm < DBH < 20 cm), square goodness-of-fit test showed that the DBH and Class V comprised old trees (DBH > 20 cm). structure of gap makers presented a common curve dis- tribution (Fig. 2). The DBH distribution of gap makers Dynamic index of gap maker structure approximated a Gaussian fit (R = 0.762, 0.749, 0.829, The dynamic structure of gap makers was quantitatively 0.896, 0.918, and 0.364, respectively; P < 0.05). However, analyzed. The dynamic index of population structure large young gap makers (DBH < 15 cm) were more com- (V ) can be represented by the change trend of gap mon in the study area, especially in the 2007–2016 Pi makers in various structures. If 0 < V ≤ 1, the quantity (Gaussian fit, R = 0.364, P < 0.05). During the investiga- Pi of gap makers increased with the V value; If − 1< V ≤ tion, we found that some young trees were injured or Pi Pi 0, the quantity of gap makers decreased with the V crushed by old trees. Pi value. The dynamic index of gap maker individuals (V ) In this study, we found that stream water was signifi- without external disturbance is described by the follow- cantly negatively correlated with tree mortality rates over ing equation: the last three decades. Based on the above data analysis, there was an ascending trend in the dynamic index of S −S n nþ1 gap maker’s population structure during the periods of V ¼  100% ð1Þ ðÞ S −S n nþ1 max 1977–1986, 1987–1996, 1997–2006, and 2007–2016, while in 1957–1966 and 1967–1976 there was a where S is the number of individuals in n DBH class, descending trend (Table 2). Furthermore, there was a S is the number of individuals in n + 1 DBH class of n +1 considerable negative interaction between the dynamic gap makers, and (…) is the maximum value in the max index value of gap maker population structure and column. Below is the formula used to calculate the dy- annual runoff; the equation of the fitted model was namic index of gap maker structure (V ): Pi y = − 0.0215x + 0.6146 (R = 0.912, P < 0.01) without ex- ternal disturbances (V ), while the equation of the 1 K−1 pi V ¼  ðÞ S  V ð2Þ P 2 pi n n K−1 n−1 fitted model was y = − 0.0005x + 0.0146 (R = 0.8405, n−1 P < 0.01) under completely random disturbances (V ′) pi where S and V are the same as in Eq. (1), K is the and y = 0.0032x +0.112 (R = 0.356, P < 0.01) under n n quantity of gap makers in different age classes, and the non-random disturbances (V ′′). V and V ′ decreased pi pi pi range of V is consistent with V . We used the formula markedly with increase of stream runoff in the study area Pi n proposed by Chen (1998) to calculate the dynamic index (Fig. 3). The V ′ value became positive and tended to pi 8 3 value under completely random (V ’) and non-random increase when the river dropped to 25.86 × 10 m . pi (V ′′) disturbances. Comparison of the DBH distribution of dead trees and pi living trees showed significant variation with a high Statistical analysis proportion of young dead trees (Fig. 4). As mentioned We have calculated species diversity according to the above, the trees surrounding CGAs and gap makers Margalef richness index and Shannon Wiener index (Ma represented living trees and dead trees, respectively. Grid 1994; Ma and Liu 1994). A fitting optimization t-test chart analysis showed that the median DBH of dead Keram et al. Forest Ecosystems (2021) 8:49 Page 6 of 13 Fig. 2 Diameter class distribution of P. euphratica gap makers in the study area trees, especially those died before 1996, were larger than habitats. P. pruinosa, P. hendersonii, C. segetum, C. auri- that of living trees. However, by comparing DBH, we culatum, A. pungens, T. mongolicum, M. platyphylla and found that the trees that died in the 1997–2016 were L. ruthenicum were rare species, which only appeared in smaller than living trees. the RBAs. P. australis, H. halodendron and A. sparsifolia grew in the CGAs, but not in the UCAs or RBAs. Forest regeneration Additionally, the RBAs contained a high number of P. Riparian vegetation was distributed on both sides of the euphratica species (30) compared to other habitats, and riverbank and was relatively simple and sparse in struc- the number of P. euphratica and other species in the ture. In our field investigation, we counted 23 seedling CGAs was higher than in the UCAs. and sapling species belonging to 12 families and 20 The seedling and sapling density of P. euphratica was genera. In general, there were obvious differences in significantly higher in the RBAs than in the CGAs and seedling and sapling regeneration among the habitats UCAs. However, compared with the UCAs, the CGAs (CGAs, UCAs, and RBAs) (Table 3). Among the 14 plant promote tree regeneration to some extent by providing fa- species found in the CGAs, 7 were also found in the vorable conditions for the survival and growth of seedling UCAs and 19 were also found in the RBAs; P. euphra- and saplings (Table 4). We also performed quantitative tica, T. ramosissima, T. hispida, T. leptostachys, G. ura- analysis of seedling and sapling regeneration, based on lensis and E. oxycarpa commonly appeared in all three species diversity and richness (Fig. 5). According to Table 2 Analysis of population structure dynamics of gap makers under different volumes of runoff water 8 −3 Years Runoff (10 m ) V V V V V PV ′ P′ V ′′ 1 2 3 4 pi pi pi 1957–1966 33.34 −0.92 0.75 0.69 −0.67 − 0.073 0.0095 − 0.0040 0.2670 −0.0180 1967–1976 31.05 −0.77 0.55 0.25 0.00 −0.060 0.0040 −0.0019 0.1430 −0.0300 1977–1986 27.85 −0.84 0.78 0.71 −0.60 0.000 0.0080 0.0025 0.5000 0.0600 1987–1996 21.83 0.87 0.80 0.33 −0.33 0.040 0.0039 0.0071 0.6350 0.0970 1997–2006 22.84 0.00 0.40 1.00 −1.00 0.200 0.0133 0.0057 0.5600 0.0400 2007–2016 8.10 −0.57 0.57 0.13 0.33 0.460 0.0154 0.0084 0.6599 0.0554 V I–II dynamic index value of I–II class sizes, V II–III dynamic index value of II–III class sizes, V III–IV dynamic index value of III–IV class sizes, V IV–V dynamic 1 2 3 4 index value of IV–V class sizes, V dynamic index value under no external disturbance, V ′ dynamic index value under completely random disturbance, V ″ pi pi pi dynamic index value of non-random disturbance, P maximum risk value under completely random disturbance, P′ maximum risk value under non-random disturbance Keram et al. Forest Ecosystems (2021) 8:49 Page 7 of 13 Fig. 3 Relationship between population structure dynamics of gap makers and annual runoff analysis of the Margalef and Shannon-Wiener indices, the C. pseudophragmites, M. platyphylla, H. glomeratu and species diversity of seedlings and saplings was remarkably E. oxycarpa, as well as some herbs such as G. glabra, I. higher in the RBAs than in the other two areas, and higher salsoloides, K. caspica, L. ruthenicum, and A. sparsifolia, in the CGAs than in the UCAs (P < 0.05). Further, though although they were very rare. the density and diversity of seedlings and saplings in the CGAs was not as high as in the RBAs, the survival rate of Discussion young trees was higher in the CGAs than in the RBAs One or more gap makers, i.e., dead trees, create canopy (Table 4). gaps in forests (Zang et al.1999). Keram et al. (2019) found that the mortality of gap makers did not substan- Comparison of gap makers and gap fillers tially contribute to increased gap size in floodplain for- The statistical analysis showed that P. euphratica is the ests along the Tarim River. This could be explained by most common species of gap maker. However, it also the small crown and DBH of gap makers. Tree mortality existed as gap filler, along with T. ramosissima and P. is caused by different agents, including drought or flood- australis (Fig. 6). Other gap fillers included P. pruinosa, ing, strong wind, diseases, and insect pests, and affects A. venetum, H. halodendron, P. hendersonii, C. segetum, forest dynamics (Krasny and Whitmore 1992; Battles C. auriculatum, A. pungens, T. mongolicum, S. collina, and Fahey 2000). Host-specific disturbance was Fig. 4 Boxplots of the DBH of dead trees and living trees on the grid charts in canopy gaps that were created at different times. The box encloses the middle 80% of observations. Median and mean values are indicated by a vertical line (−) and a star (★), respectively. The upper whisker indicates the largest DBH value, while the lower whisker shows the smallest. The plus sign (+) indicates the maximum DBH Keram et al. Forest Ecosystems (2021) 8:49 Page 8 of 13 Table 3 Seedling and sapling recruitment in different habitats strong winds or insect pests may also play a role (Han in the study area et al. 2011). Species CGAs UCAs RBAs Water is the most important limiting factor for the maintenance and growth of P. euphratica in arid ecosys- P. euphratica 6330 tems, and P. euphratica depends on ground water that P. pruinosa –– 5 available to its roots (Yu et al. 2019). It is known that T. ramosissima 10 5 11 water is the factor with most influence on the survival, T. hispida 324 distribution, and development of vegetation in hyper- T. leptostachya 213 arid regions. In arid ecosystems, the vitality of P. euphra- P. australis 10 –– tica forests strongly depend on the groundwater level (Halik et al. 2006, 2009; Wang et al. 2008; Ginau et al. H. halodendron 2 –– 2013; Keyimu et al. 2017). Halik et al. (2019) reported G. uralensis 524 that the vitality of P. euphratica diminished when the A. sparsifolia 2 –– groundwater level was not sufficient for the development H. glomeratus –– 2 of tree stands. In addition, Xu et al. (2016) studied the K. caspica 3 – 4 correlation between groundwater and surface water I. salsoloides 1 – 2 using stable isotope technology. Their results indicated that the groundwater level is significantly affected by P. hendersonii –– 1 surface water. Chen et al. (2004) and Keilholz et al. C. segetum –– 2 (2015) also demonstrated that groundwater is often sup- C. auriculatum –– 3 plied by stream flow and infiltration from watercourses. A. pungens –– 2 The groundwater depth in the Tarim Basin has dropped S. collina 2 – 4 continuously as a result of the desiccation of streams C. pseudophragmites 4 – 5 and decreases in stream flow since an embankment was built in 2000. Thus, the physical characteristics of plant T. mongolicum –– 3 habitats have been changed. Consequently, the growth M. platyphylla –– 4 of P. euphratica cannot be sustained as before in this re- L. ruthenicum –– 2 gion, and forests have highly degraded or died. Accord- E. oxycarpa 113 ing to the present study, hydrological alterations may be A. venetum – 3 – the main cause of tree death in desert riparian forests CGAs Canopy gap areas, UCAs Undercanopygap areas, RBAs Riverbank areas along the Tarim River. DBH is one of the most common forest inventory variables and can reflect the population structure and growth status of tree stands (Condit et al. considered to be the most common agent of tree mortal- 2000; Di et al. 2014). Due to the unique climate and ity in old-growth mixed beech forests in the Western habitat conditions of desert floodplain areas, tree shape Carpathians (Orman and Dorota 2017), while wind dis- variables are obviously different from those in temperate turbance led to a disproportionate number of gap or/and subtropical forests (Ling et al. 2015). In the makers in old-growth red spruce–Fraser fir forests in present research, it was found that most young gap North Carolina (White et al. 1985) and red spruce–bal- makers (DBH < 15 cm) died in the most recent two de- sam stands in New Hampshire (Foster and Reiners 1986; cades, i.e., 1997–2006 and 2007–2016. In the early Worrall et al. 2005). However, the causes of tree mortal- 1970s, water resources were affected on a regional scale ity in riparian forests are different from those in other by climate change and anthropogenic activities (in- forest types, such as tropical and subtropical forests. The creased water use for irrigation), and then lag effects led results of the present study showed that tree mortality in to a progressive decrease in surface water from 1984 and riparian forests along the Tarim River was mainly influ- caused a reduction in groundwater from 2004 (Keram enced by hydrological factors (surface water), although et al. 2019). This resulted in habitat deterioration and a Table 4 Comparison of P. euphratica seedlings and saplings in different habitats in the middle reaches of the Tarim River Plot Occurrence Seedling and sapling Plant height (cm) Young tree density Plant height (cm) −2 −2 frequency (%) density (Plant∙m ) (Plant∙m ) CGAs 45.63 0.12 ± 0. 06b 3.53 ± 0.86b 8.31 ± 1.64a 285.0 ± 1.62a UCAs 34.52 0.08 ± 0.03b 3.12 ± 0.45b 0.28 ± 0.06b 167.0 ± 1.55b RBAs 100.00 17.56 ± 3.12a 18.76 ± 3.95a 0.34 ± 0.04b 187.0 ± 1.51b CGAs canopy gap areas, UCAs undercanopy areas, RBAs riverbank areas Keram et al. Forest Ecosystems (2021) 8:49 Page 9 of 13 Fig. 5 Species diversity of seedlings and saplings in different habitats decline in the survival of young P. euphratica individ- by the hydrological conditions, and only a small propor- uals. Based on quantitative correlation analysis, the tion of young trees survived to reach canopy height. present study revealed that gap maker mortality and However, there were a large number of larger, mature DBH structure responded negatively to variations in sur- trees among the living trees that have developed root face water. This may be due to long-lasting exogenous systems and are therefore not significantly affected by disturbances, such as a scarcity of surface water or a de- the reduced groundwater level. crease in groundwater level, having a notable influence By creating environmental heterogeneity in terms of on vegetation through drought stress. Thus, the water light availability, gap disturbances play a key role in requirement of riparian vegetation was not satisfied. forest regeneration as well as in the establishment and Therefore, gap makers of low DBH appeared frequently development of tree species with different ecological in P. euphratica forests, which experienced an increase recruitment patterns (Runkle 1989; Peterken 1996; in more resilient species. This may be because the vital- Mountford 2006; Zhang et al. 2006; Li et al. 2013). Gap- ity of P. euphratica in this arid area was directly affected based restoration provides a flexible system that Fig. 6 Comparison of gap maker and gap filler species in the middle reaches of the Tarim River Keram et al. Forest Ecosystems (2021) 8:49 Page 10 of 13 emulates natural regeneration under different environ- CGAs, and lowest in the UCAs. It may have been af- ments. Van and Dignan (2007) reported that seedling fected by light availability and hydrological conditions and sapling diversity and richness were promoted by during seedling and sapling recruitment. light intensity. Zhu et al. (2014) stated in a review of In conclusion, searching the tree mortality and plant published studies that compared to UCAs, the seedlings regeneration are considered to be the most effective and saplings of shade-tolerant species have significantly ways to clarify the development trend of desert riparian higher density in CGAs. Overall, water, light availability, forest. Furthermore, the protection and maintenance of and seedbed conditions potentially affect seedling emer- the floodplain vegetation along the Tarim River are cru- gence, germination, and establishment in floodplain ri- cial to sustainable regional development in the Tarim parian forests. It was found that appearance of seedling Basin. Our study area is located in the Yingbazha is closely related to flooding patterns in this study. Be- section, where there are fragment pits of stagnant water sides, a higher density of seedlings and saplings is ex- by flooding. Moreover, though the density and diversity pected in gaps as they provide full light conditions. The of seedlings and saplings in the CGAs was not as high as results of the present observations showed that the seed- in the RBAs, the survival rate of young trees was higher ling and sapling species diversity was higher in the CGAs in the CGAs than in the RBAs. Accordingly, we suggest than in the UCAs. Hence, forest canopy gaps playing a that making full use of limited resources of stagnant pivotal role in the long-term germination and regener- water pits to establish a “Natural Nursery of the Poplar” ation of plant species in some degree. Oliver and Larson and appropriately transplanted into the gap, the main (1996) stated that canopy gaps simultaneously result in purpose of which is to achieve an improved regeneration mortality in some individuals and establishment and efficiency of the Populus euphratica riparian forests. growth in others, and the ongoing process of death and Additionally, there is a need for further applied experi- replacement has a profound effect on forest structure ment to establish test areas for flood water allocation and composition. Pham et al. (2004) showed that Abies and utilization strategies, which, if successful, should balsamea is the most frequent successor in Abies forest then be extended to the whole basin. Thus, we argue stands and that Picea mariana is the most likely replace- that, to some degree, canopy gaps can provide a favor- ment species in Picea forest stands, regardless of the able environment for seedlings to grow into saplings, species of gap makers. In the present study, canopy gaps thereby reaffirming the importance of gap creation for were filled with shrubs and herbs. When we examined forest regeneration. the probability of replacement in floodplain forests, there Abbreviations seemed to be a reciprocal replacement of P. euphratica by DRB: Distance from river bank; GWL: Groundwater level; DBH: Diameter at T. ramosissima.and P. australis (Fig. 6). However, the the breast height; TH: Tree height; CGAs: Canopy gap areas; probability of P. euphratica replacement was lower when UCAs: Undercanopy areas; RBAs: Riverbank areas; THNR: Tarim Huyanglin nature reserve the mortality of young trees was particularly high. Acknowledgments Conclusion and suggestion Cordial thanks to MSc-Students (Elyar, Wang Wenjuan, Ramile, Mihray and As the largest riparian forest ecosystem in Northwest Gao Qing) from the Xinjiang University for their intensive support during the field investigation. Special thanks to Xinjiang LIDAR Applied Engineering China, Euphrates poplar forests are facing great chal- Technology Research Center for giving us free use of the Riegl VZ-1000 Ter- lenges of climate change and human interference (Zhang restrial Laser Scanner. We thank the anonymous reviewers for their construct- et al. 2016; Halik et al. 2019). Monitoring of tree mortal- ive comments that greatly helped us improve the quality of this manuscript. ity and plant regeneration in different habitats is consid- Authors’ contributions ered the most effective way of ensuring the sustainable Ayjamal Keram conceived, designed and performed the experiments, regeneration and ecological restoration of floodplain for- contributed the data collection, analyzed the data, prepared figures and ests along the Tarim River. Based on evaluation of the tables, and authored or approved the final draft of the manuscript. Ümüt Halik conceived and designed the experiments, contributed the data correlation between tree mortality and water resources collection, reviewed the manuscript, and approved the final draft. Tayierjiang in the study area, hydrological conditions (runoff vol- Aisahan, Maierdang Keyimu and Kadeliya Jiapaer participated in field work, ume) are the main reason for the death of P. euphratica contributed the data collection/validation and language proofreading. Guolei Li edited and validated the manuscript with critical comments and reviewed forests. At the beginning of the 1970s, hydrological con- the manuscript draft. All authors checked and approved the final manuscript. ditions were affected on a regional scale by anthropo- genic activities. Consequently, riparian forests were Funding threatened under water scarcity, which resulted in high The work was funded by the National Natural Science Foundation of China (31860134, U1703102, 31700386). mortality among Populus species. Our observations of seedling and sapling density indicated that the frequency Availability of data and materials of plant species was different among the three studied The data set generated for the study area is available from the habitats; it was highest in the RBAs, intermediate in the corresponding author on reasonable request. Keram et al. Forest Ecosystems (2021) 8:49 Page 11 of 13 Declarations Droessler L, Von LB (2005) Canopy gaps in two virgin beech forest reserves in Slovakia. J Sci 51(10):446–457. https://doi.org/10.17221/4578-JFS Ethics approval and consent to participate Foetzki A (2004) Transpiration and sap flow in Alhagi sparsifolia, Calligonum Not applicable. caput-medusae, Populus euphratica and Tamarix ramosissima. In: Runge M, Zhang X (eds) Ecophysiology and habitat requirements of perennial plant species in the Taklimakan Desert. Shaker Press, Aachen, pp 67–74 Consent for publication Foster JR, Reiners WA (1986) Size distribution and expansion of canopy gaps in a Not applicable. northern Appalachian spruce-fir forest. Vegetatio 68(2):109–114 Ginau A, Opp C, Sun Z, Halik Ü (2013) Influence of sediment, soil, and micro-relief Competing interests conditions on vitality of Populus euphratica stands in the lower Tarim riparian The authors declare that they have no competing interests. ecosystem. Quatern Intern 311:146–154. https://doi.org/10.1016/j.quaint.2013. 06.025 Author details Gries D, Zeng F, Foetzki A, Arndt SK, Bruelheide H, Thomas FM (2003) College of Resources and Environmental Science, Key Laboratory for Oasis Growth and water relations of Tamarix ramosissima and Populus Ecology of Ministry of Education, Xinjiang University, Ürümqi 830046, China. euphratica on Taklamakan desert dunes in relation to depth to a College of Forestry, Key Laboratory for Silviculture and Conservation of permanent water table. Plant Cell Environ 26(5):725–736. https://doi.org/1 Ministry of Education, Beijing Forestry University, Beijing 100083, China. 0.1046/j.1365-3040.2003.01009.x State Key Laboratory of Urban and Regional Ecology, Research Center for Halik Ü, Aishan T, Betz F, Kurban A, Rouzi A (2019) Effectiveness and challenges Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, of ecological engineering for desert riparian forest restoration along China’s China. largest inland river. Ecol Eng 127:11–22. https://doi.org/10.1016/j.ecoleng.201 8.11.004 Received: 26 February 2021 Accepted: 9 June 2021 Halik Ü, Chai Z, Kurban A, Cyffka B (2009) The positive response of some ecological indices of Populus euphratica to the emergency water transfer in the lower reaches of the Tarim River. Res Sci 31:1309–1314. https://doi.org/1 0.1007/978-3-642-00205-2_9 References Halik Ü, Kurban A, Mijit M, Schulz J, Parroth F, Coenradie B (2006) The potential Aishan T (2016) Degraded Tugai forests under rehabilitation in the Tarim riparian influence of embankment engineering and ecological water transfer on the ecosystem, Northwest China: monitoring, assessing and modeling. KU riparian vegetation along the middle and lower reaches of the Tarim River. Eichstätt-Ingolstadt, Germany In: Hoppe T, Kleinschmit B, Roberts B, Thevs N, Halik Ü (eds) Watershed and Aishan T, Betz F, Halik Ü, Cyffka B, Rouzi A (2018) Biomass carbon sequestration floodplain management along the Tarim River in China’s arid northwest. potential by riparian forest in the Tarim River watershed, Northwest China: Shaker Press, Aachen, pp 221–236 implication for the mitigation of climate change impact. Forests 9(4):196– Han L, Wang H, Chen J (2011) Disturbance regimes and gaps characteristics of 211. https://doi.org/10.3390/f9040196 the desert riparian forest at the middle reach of Tarim River. Acta Ecol Sin Albanesi E, Gugliotta OI, Mercurio L, Mercurio R (2005) Effects of gap size and 31(16):4699–4708. https://doi.org/10.1007/s11676-011-0113-8 within-gap position on seedlings establishment in silver fir stands. iForest Han WJ, He JF, Zhang WH, Li JX (2013) Effects of gap size on root growth of 1(1):55–59. https://doi.org/10.3832/ifor0448-0010055 seedlings and soil physical and chemical properties in plantation in the Battles JJ, Fahey TJ (2000) Gap dynamics following forest decline: a case study of Huanglong forest. Sci Silv Sin 11(49):17–23 red spruce forests. Ecol Appl 10(3):760–774. https://doi.org/10.2307/2641043 Hu S, Zhao R, Tian C, Song Y (2009) Empirical models of calculating phreatic Betz F, Halik Ü, Kuba M, Aishan T, Cyffka B (2015) Controls on aeolian sediment evaporation from bare soil in Tarim River basin, Xinjiang. Environ Geol 59(3): dynamics by natural riparian vegetation in the eastern Tarim Basin, Northwest 663–668. https://doi.org/10.1007/s12665-009-0063-z China. Aeolian Res 18:23–34. https://doi.org/10.1016/j.aeolia.2015.04.005 Huang P (2002) Irrigation-free vegetation and its recovery in arid region. Science Bottero A, Garbarino M, Dukić V, Govedar Z, Lingua E, Nagel TA (2011) Gap-phase Press, Beijing, pp 15–50 dynamics in the old-growth forest of Lom, Bosnia and Herzegovina. Silv Huang T, Pang Z (2011) Changes in groundwater induced by water diversion in Fenn 45(5):875–887. https://doi.org/10.14214/sf.76 the lower Tarim River, Xinjiang Uygur, NW China: evidence from Chen XD (1998) A study on the method of quantitative analysis for plant environmental isotopes and water chemistry. J Hydrol 387(3–4):188–201. population and community structural dynamics. Acta Ecol Sin 18(2):214–217 https://doi.org/10.1016/j.jhydrol.2010.04.007 Chen YJ, Li WH, Liu JZ, Yang YH (2013) Effects of water conveyance Huang X, Chen YN, Ma JX, Chen YP (2010) Study on change in value of embankments on riparian forest communities at the middle reaches of the ecosystem service function of Tarim River. Acta Ecol Sin 30(2):67–75. https:// Tarim River, Northwest China. Ecohydrology 6(6):937–948. https://doi.org/10.1 doi.org/10.1016/j.chnaes.2010.03.004 002/eco.1418 Jankovska I, Brūmelis G, Nikodemus O, Kasparinskis R, Amatniece V, Straupmanis Chen YN, Wang Q, Ruan X, Li WH, Chen YP (2004) Physiological response of G (2015) Tree species establishment in urban forest in relation to vegetation Populus euphratica to artificial water-recharge of the lower reaches of the composition, tree canopy gap area and soil factors. Forests 6(12):4451–4461. Tarim River. Acta Bot Sin 46(12):1393–1401 https://doi.org/10.3390/f6124379 China Green Foundation (2018) Ecological Restoration Plan of Populus euphratica Keilholz P, Disse M, Halik Ü (2015) Effects of land use and climate change on Forest Along the Belt and Road. China Forestry Publishing House, Beijing groundwater and ecosystems at the middle reaches of the Tarim River using Coates KD, Philip JB (1997) A gap-based approach for development of the MIKE SHE integrated hydrological model. Water 7(12):3040–3056. https:// silvicultural systems to address ecosystem management objectives. Forest doi.org/10.3390/w7063040 Ecol Manag 99(3):337–354. https://doi.org/10.1016/S0378-1127(97)00113-8 Keram A, Halik Ü, Keyimu M, Aishan T, Mamat Z, Rouzi A (2019) Gap dynamics of Condit R, Ashton PS, Baker P, Bunyavejchewin S (2000) Spatial patterns in the natural Populus euphratica floodplain forests affected by hydrological distribution of tropical tree species. Science 288(5470):1414–1418. https://doi. alteration along the Tarim River: implications for restoration of the riparian forests. org/10.1126/science.288.5470.1414 Forest Ecol Manag 438:103–113. https://doi.org/10.1016/j.foreco.2019.02.009 Deng MJ (2009) Theory and practice of water management in the Tarim River, Kern CC, Burton JI, Raymond P, D'Amato AW, Keeton WS (2017) Challenges China. Science Press, Beijing facing gap-based silviculture and possible solutions for Mesic northern Deng MJ (2016) Prospecting development of southern Xinjiang: water strategy forests in North America. Forestry 90(1):4–17. https://doi.org/10.1093/forestry/ and problem of Tarim River basin. Arid Land Geog 39(1):1–11 cpw024 Di K, Guo YX, Ren CJ, Zhao FZ, Feng YZ, Han XH, Yang GH (2014) Population Kern CC, Montgomery RA, Reich PB, Strong TF (2013) Canopy gap size influences structure and spatial pattern of main tree species in secondary Betula niche partitioning of the ground-layer plant community in a northern platyphylla forest in Ziwuling Mountains, China. Sci Rep 4(1):6873. https://doi. temperate forest. J Plant Ecol 6(1):101–112. https://doi.org/10.1093/jpe/rts016 org/10.1038/srep06873 Dorotä D, Thomast V (2008) Tree fall-gap structure and regeneration in mixed Keyimu M, Halik Ü, Rouzi A (2017) Relating water use to tree vitality of Populus Abies alba stands in Central Poland. Forest Ecol Manag 255(8):3469–3476. euphratica Oliv. In the lower Tarim River, NW China. Water 8(9):622. https:// https://doi.org/10.1016/j.foreco.2008.02.025 doi.org/10.3390/w9080622 Keram et al. Forest Ecosystems (2021) 8:49 Page 12 of 13 Keyimu M, Halik Ü, Wumaier G, Abliz A (2018) Diel variation of Populus euphratica Rentch JS, Schuler TM, Nowacki GJ, Beane NR, Ford WM (2010) Canopy gap sap flow and its correlation with meteorological factors. J Glaciol Geocryol dynamics of second-growth red spruce-northern hardwood stands in West 40(1):166–175 Virginia. Forest Ecol Manag 260(10):1921–1929. https://doi.org/10.1016/j. Kitao M, Kitaoka S, Harayama H (2018) Canopy nitrogen distribution is optimized foreco.2010.08.043 to prevent photo inhibition throughout the canopy during sun flecks. Sci Rouzi A, Halik Ü, Thevs N, Aishan T (2018) Water efficient alternative crops for Rep 8(1):503–513. https://doi.org/10.1038/s41598-017-18766-0 sustainable agriculture along the Tarim Basin: a comparison of the economic potentials of Apocynum pictum, Chinese red date and cotton in Xinjiang, Krasny ME, Whitmore MC (1992) Gradual and sudden forest canopy gaps in China. Sustainability 10(1):1–17. https://doi.org/10.3390/su10010035 Allegheny northern hardwood forests. Can J Forest Res 22(2):139–142. https://doi.org/10.1139/x92-019 Rumbaur C, Thevs N, Disse M (2015) Sustainable management of river oases Lang P, Ahlborn J, Schäfer P, Wommelsdorf T, Jeschke M, Zhang X, Thomas FM along the Tarim River (SuMaRiO) in northwestern China under conditions of (2016) Growth and water use of Populus euphratica trees and stands with climate change. Earth Sys Dynam 5(2):1221–1273. https://doi.org/10.5194/ different water supply along the Tarim River, Northwest China. Forest Ecol esd-6-83-2015 Manag 380:139–148. https://doi.org/10.1016/j.foreco.2016.08.049 Runkle JR (1989) Synchrony of regeneration, gaps, and latitudinal differences in Li ZP, Li WM, Wu FZ, Yang WQ, Xu ZF, Tan P, Wu QG (2013) Effects of forest gap tree species diversity. Ecology 70(3):546–547. https://doi.org/10.2307/1940199 on content of soil nit rate nitrogen and ammonium nitrogen at different Runkle JR (1998) Changes in southern Appalachian canopy tree gaps sampled critical periods of subalpine forest in Western Sichuan. J Soil Water Conserv thrice. Ecology 79(5):1768–1780. https://doi.org/10.2307/176795 6(27):270–274 Runkle JR, Yetter TC (1987) Treefalls revisited: gap dynamics in the southern Ling H, Zhang P, Xu H, Zhao X (2015) How to regenerate and protect desert Appalachians. Ecology 68(2):417–424. https://doi.org/10.2307/1939273 riparian Populus euphratica forest in arid areas. Sci Rep 5(154):81–89. https:// Schliemann SA, Bockheim JG (2011) Methods for studying tree fall gaps: a doi.org/10.1038/srep15418 review. Forest Ecol Manag 261(7):1143–1151. https://doi.org/10.1016/j. Liu Q, Hytteborn H (1991) Gap structure, disturbance and regeneration in a foreco.2011.01.011 primeval Picea abies forest. J Veg Sci 2(3):391–402. https://doi.org/10.23 Sharma LN, Shrestha KB, Maren IE (2018) Tree regeneration in gap-understory 07/3235932 mosaics in a subtropical shorea robusta (Sal) forest. J Forest Res 30(6):2061– Lu DL, Wang GG, Yan QL, Gao T, Zhu JJ (2018) Effects of gap size and within-gap 2068. https://doi.org/10.1007/s11676-018-0747-x position on seedling growth and biomass allocation: is the gap partitioning Song YD, Fan ZL, Lei ZD, Zhang FW (2000) Research on water resources and hypothesis applicable to the temperate secondary forest ecosystems in ecology of Tarim River, China. Xinjiang People’s Press, China, Beijing Northeast China? Forest Ecol Manag 429:351–362. https://doi.org/10.1016/j. Thevs N (2006) Ecology, spatial distribution, and utilization of the Tugai foreco.2018.07.031 vegetation at the middle reaches of the Tarim River, Xinjiang, China. Ernst- Ma KP (1994) Measurement of biotic community diversity: I. a diversity (part. 1). Moritz-Arndt University of Greifswald, Germany Biodivers Sci 2(3):162–168. https://doi.org/10.17520/biods.1994027 Thevs N, Zerbe S, Schnittler M, Abdusalih N, Succow M (2008) Structure, Ma KP, Liu YM (1994) Measurement of biotic community diversity: I. a reproduction and flood-induced dynamics of riparian Tugai forests at the diversity (part. 2). Biodive Sci 2(4):231–239. https://doi.org/10.17520/ Tarim River in Xinjiang, NW China. Forestry 81(1):45–57. https://doi.org/10.1 biods.1994027 093/forestry/cpm043 Mamat Z, Halik Ü, Aishan T, Aini A (2019) Ecological effect of the riparian Thomas AN, Jurij D (2006) Intermediate wind disturbance in an old-growth ecosystem in the lower reaches of the Tarim River in Northwest China. Plos beech-fir forest in south eastern Slovenia. Can J For Res 36(3):629–638. One 14(1):e0208462. https://doi.org/10.1371/journal.pone.0208462 https://doi.org/10.1139/x05-263 Mamat Z, Halik Ü, Keyimu M, Keram A, Nurmamat K (2018) Variation of the Thomas FM, Lang P (2020) Growth and water relations of riparian poplar forests floodplain forest ecosystem service value in the lower reaches of the Tarim under pressure in Central Asia's Tarim River basin. River Res Applic 37(2):233– River, China. Land Degrad Dev 29(1):47–57. https://doi.org/10.1002/ldr.2835 248. https://doi.org/10.1002/rra.3605 Mountford EP (2006) Patterns of regeneration and ground vegetation associated Thomas FM, Yu R, Schäfer P, Zhang XM, Petra L (2017) How diverse are Populus with canopy gaps in a managed beechwood in southern England. Forestry “diversifolia” leaves? Linking leaf morphology to eco-physiological and stand 79(4):389–408. https://doi.org/10.1093/forestry/cpl024 variables along water supply and salinity gradients. Flora 233:68–78. https:// Nagel TA, Svoboda M (2008) Gap disturbance regime in an old-growth Fagus doi.org/10.1016/j.flora.2017.05.007 abies forest in the Dinaric Mountains, Bosnia-Herzegovina. Can J For Res Van PJD, Dignan P (2007) Regeneration after 8 years in artificial canopy gaps in 38(11):2728–2737. https://doi.org/10.1139/X08-110 mountain ash (Eucalyptus regnans) forest in south-eastern Australia. Forest Nagel TA, Svoboda M, Kobal M (2016) Disturbance, life history traits, and Ecol Manag 244(1):102–111. https://doi.org/10.1016/j.foreco.2007.03.055 dynamics in an old-growth forest landscape of southeastern Europe. Ecol Walker LR (2000) Seedling and sapling dynamics of treefall pits in Puerto Rico1. Appl 24(4):663–679. https://doi.org/10.1890/13-0632.1 Biotropica 32(2):262–275. https://doi.org/10.1111/j.1744-7429.2000.tb00469.x Nagel TA, Svoboda M, Rugani T, Diaci J (2010) Gap regeneration and Wang JS, Halik Ü, Cyffka B, Kurban O, Peng SH (2008) Study on DBH-structure of replacement patterns in an old-growth Fagus-Abies forest of Bosnia- Populus euphratica and their spatial distribution in the lower reaches of the Herzegovina. Plant Ecol 208(2):307–318. https://doi.org/10.1007/s11258-009- Tarim River. Chin Bull Bot 25(6):728–733. https://doi.org/10.3969/j.issn.1674-34 9707-z 66.2008.06.014 Oliver CD, Larson BC (1996) Forest stand dynamics. Willey, New York Wang SJ, Chen BH, Li HQ (1995) Euphrates poplar forest, 1st end. China Environ Orman O, Dorota D (2017) Gap dynamics in the Western Carpathian mixed Sci Press, Beijing beech old growth forests affected by spruce bark beetle outbreak. Eur J Wen K (2016) Study on the relationship among Abies faxoniana regeneration, Forest Res 136(4):571–581. https://doi.org/10.1007/s10342-017-1054-3 canopy gaps, and Fargesia denudate in subalpine coniferous forest in Peterken GF (1996) Natural woodland: ecology and conservation in northern southwestern Sichuan, China. Beijing Forestry University, Beijing temperate regions (2001 reprint). J Ecol 84(5):790. https://doi.org/10.2307/22 White PS, Mackenzie MD, Busing RT (1985) Natural disturbance and gap phase dynamics in southern Appalachian spruce-fir forests. Can J For Res 15(1):233– Petritan AM, Nuske RS, Petritan IC (2013) Gap disturbance patterns in an old- 240. https://doi.org/10.1139/x85-041 growth sessile oak (Quercus petraea L.), European beech (Fagus sylvatica L.) Worrall JJ, Lee TD, Harrington TC (2005) Forest dynamics and agents that initiate forest remnant in the Carpathian Mountains, Romania. Forest Ecol Manag and expand canopy gaps in Picea-abies forests of Crawford notch, New 308(4):67–75. https://doi.org/10.1016/j.foreco.2013.07.045 Hampshire, USA. J Ecol 93(1):178–190. https://doi.org/10.1111/j.1365-2745.2 Pham AT, Grandpré LD, Gauthier S, Bergeron Y (2004) Gap dynamics and 004.00937.x replacement patterns in gaps of the northeastern boreal forest of Quebec. Xu C, Chen Y, Li W (2006) Climate change and hydrologic process response in Can J For Res 34(2):353–364. https://doi.org/10.1139/x03-265 the Tarim River basin over the past 50 years. Chin Sci Bull 51(1):25–36. Popa I, Nechita C, Hofgaard A (2017) Stand structure, recruitment and growth https://doi.org/10.1007/s11434-006-8204-1 dynamics in mixed subalpine spruce and Swiss stone pine forests in the Xu MC, Chen YJ, Liu JZ, Wang D, Liu YQ (2016) The distribution pattern dynamics eastern Carpathians. Sci Total Environ 598:1050–1057. https://doi.org/10.1016/ of Populus euphratica are influenced by hydrology in the middle reaches of j.scitotenv.2017.04.169 Tarim River. Acta Ecol Sin 36(9):2646–2655 Keram et al. Forest Ecosystems (2021) 8:49 Page 13 of 13 Yamamoto SI (2000) Forest gap dynamics and tree regeneration. J Forest Res 5(4):223–230. https://doi.org/10.1007/BF02767114 Yang H, Liu SR, Cao KF, Wang JX, Li YD, Xu H (2017) Characteristics of typhoon disturbed gaps in an old-growth tropical montane rainforest in Hainan Island, China. J Forest Res 28(6):1231–1239. https://doi.org/10.1007/s11676-01 7-0402-y Yu HY, Ya NC, We HL (2009) Relationship between soil properties and plant diversity in a desert riparian forest in the lower reaches of the Tarim River, Xinjiang, China. Arid Soil Res Rehab 23(4):283–296. https://doi.org/10.1080/1 Yu TF, Feng Q, Si JH, Xi HY, O'Grady AP, Pinkard EA (2019) Responses of riparian forests to flood irrigation in the hyper-arid zone of NW China. Sci Total Environ 648:1421–1430. https://doi.org/10.1016/j.scitotenv.2018.08.287 Zang RG, Liu JY, Dong DF (1999) Gap dynamics and forest biodiversity. China Forestry Publishing House, Beijing, pp 1–242 Zeng F, Zhang X, Foetzki A, LiX RM (2002) Study on the water physiological characteristics of Populus euphratica in the Cele oasis. Arid Zone Res 19(2): 26–30 Zeng FJ, Bleby TM, Landman PA (2006) Water and nutrient dynamics in surface roots and soils are not modified by short-term flooding of phreatophytic plants in a hyperarid desert? Plant Soil 279(1–2):129–139. https://doi.org/10.1 007/s11104-005-0498-2 Zeng FJ, Zhang WJ, Liu GJ, Zhang DY, Li XY, Zhang L, Yuan LM, Zhang XM (2020) Stable restoration pattern and sustainable management technology of main dominant vegetation in typical desert areas of China. Bull Chin Acad Sci 214(6):63–70 Zhang CY, Zhao XH, Zheng JM (2006) A study on soil properties in forest gaps and under canopy in broad-leaved Pinus koraiensis forest in Changbai Mountain. Forest Res 19(3):347–352 Zhang Q, Gu XH, Singh VP (2016) Magnitude, frequency and timing of floods in the Tarim River basin, China: changes, causes and implications. Glob Planet Chang 139:44–55. https://doi.org/10.1016/j.gloplacha.2015.10.005 Zhao ZY, Zhang K, Lu L (2011) Seedling recruitment in desert riparian forest following river flooding in the middle reaches of the Tarim River. Acta Ecol Sin 31(12):3322–3329. https://doi.org/10.3724/SP.J.1011.2011.00338 Zhu JJ, Lu DL, Zhang WD (2014) Effects of gaps on regeneration of woody plants: a meta-analysis. J Forest Res 25(3):501–510. https://doi.org/10.1007/ s11676-014-0489-3 Zhu JJ, Mao ZH, Hu LL, Zhang JX (2007) Plant diversity of secondary forests in response to anthropogenic disturbance levels in montane regions of northeastern China. J Forest Res 12(6):403–416. https://doi.org/10.1007/s1031 0-007-0033-9 Zhu JJ, Matsuzaki T, Li FQ (2003) Effect of gap size created by thinning on seedling emergency, survival and establishment in a coastal pine forest. Forest Ecol Manag 182(1–3):339–354. https://doi.org/10.1016/S0378-1127(03 )00094-X Zhu JJ, Yan Q, Yu LZ, Zhang J, Yang K, Gao T (2018) Support ecological restoration and sustainable management of forests in Northeast China based on research of forest ecology and demonstrations. Bull Chin Acad Sci 33(1): 107–118 Zhu JJ, Zhang GQ, Wang GG (2015) On the size of forest gaps: can their lower and upper limits be objectively defined. Agric For Meteorol 213(2):64–76. https://doi.org/10.1016/j.agrformet.2015.06.015 Zhu JJ, Zhu CY, Lu DL, Wang GG, Zheng X, Cao JS, Zhang JX (2021) Regeneration and succession: a 50-year gap dynamic in temperate secondary forests, Northeast China. Forest Ecol Manag 484:118943. https://doi.org/10.1016/j. foreco.2021.118943

Journal

"Forest Ecosystems" – Springer Journals

Published: Jul 22, 2021

Keywords: Tree mortality; Regeneration strategy; Seedling and sapling recruitment; Gap makers; Riparian forest; Tarim River

There are no references for this article.